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Regexp::Grammars - Add grammatical parsing features to Perl 5.10 regexes


This document describes Regexp::Grammars version 1.001_005


    use Regexp::Grammars;

    my $parser = qr{
            <Verb>               # Parse and save a Verb in a scalar
            <.ws>                # Parse but don't save whitespace
            <Noun>               # Parse and save a Noun in a scalar

            <type={ rand > 0.5 ? 'VN' : 'VerbNoun' }>
                                 # Save result of expression in a scalar
                <[Noun]>         # Parse a Noun and save result in a list
                                     (saved under the key 'Noun')
                <[PostNoun=ws]>  # Parse whitespace, save it in a list
                                 #   (saved under the key 'PostNoun')

            <Verb>               # Parse a Verb and save result in a scalar
                                     (saved under the key 'Verb')

            <type={ 'VN' }>      # Save a literal in a scalar
            <debug: match>       # Turn on the integrated debugger here
            <.Cmd= (?: mv? )>    # Parse but don't capture a subpattern
                                     (name it 'Cmd' for debugging purposes)
            <[File]>+            # Parse 1+ Files and save them in a list
                                     (saved under the key 'File')
            <debug: off>         # Turn off the integrated debugger here
            <Dest=File>          # Parse a File and save it in a scalar
                                     (saved under the key 'Dest')


        <token: File>              # Define a subrule named File
            <.ws>                  #  - Parse but don't capture whitespace
            <MATCH= ([\w-]+) >     #  - Parse the subpattern and capture 
                                   #    matched text as the result of the
                                   #    subrule

        <token: Noun>              # Define a subrule named Noun
            cat | dog | fish       #  - Match an alternative (as usual) 

        <rule: Verb>               # Define a whitespace-sensitive subrule
            eats                   #  - Match a literal (after any space)
            <Object=Noun>?         #  - Parse optional subrule Noun and
                                   #    save result under the key 'Object'
        |                          #  Or else...
            <AUX>                  #  - Parse subrule AUX and save result
            <part= (eaten|seen) >  #  - Match a literal, save under 'part'

        <token: AUX>               # Define a whitespace-insensitive subrule
            (has | is)             #  - Match an alternative and capture
            (?{ $MATCH = uc $^N }) #  - Use captured text as subrule result


    # Match the grammar against some text...
    if ($text =~ $parser) {
        # If successful, the hash %/ will have the hierarchy of results...
        process_data_in( %/ );


In your program...

    use Regexp::Grammars;    Allow enhanced regexes in lexical scope
    %/                       Result-hash for successful grammar match

Defining rules in your grammar...

    <rule:     IDENTIFIER>   Define rule with magic whitespace
    <token:    IDENTIFIER>   Define rule without magic whitespace
    <objrule:  IDENTIFIER>   Define rule returning blessed result-hash
    <objtoken: IDENTIFIER>   Define token returning blessed result-hash

Matching rules in your grammar...

    <RULENAME>               Call named subrule,
                             save result to $MATCH{RULENAME}

    <%HASH>                  Match longest possible key of hash

    <ALIAS= RULENAME>        Call subrule, save result in $MATCH{ALIAS}
    <ALIAS= %HASH>           Match a hash key, save in $MATCH{ALIAS}
    <ALIAS= ( PATTERN )>     Match pattern, save match in $MATCH{ALIAS}
    <ALIAS= (?{ CODE })>     Execute code, save value in $MATCH{ALIAS}

    <.SUBRULE>               Call any kind of subrule (as above),
                             but don't save the result in %MATCH

    <[SUBRULE]>              Call any kind of subrule (as above) but
                             append result instead of overwriting it

    <SUBRULE1> ** <SUBRULE2> Match one or more repetitions of SUBRULE1
                             as long as they're separated by SUBRULE2

In your grammar's code blocks...

    $CAPTURE    Alias for $^N (the most recent paren capture)
    $CONTEXT    Another alias for $^N
    $INDEX      Current index of next matching position in string
    %MATCH      Current rule's result-hash
    $MATCH      Magic override value (returned instead of result-hash)
    $DEBUG      Current match-time debugging mode

Debugging support...

    <require: (?{ CODE })>   Fail if code evaluates false
    <debug: COMMAND >        Change match-time debugging mode
    <error: TEXT|CODEBLOCK>  Queue text or value as an error message
    <logfile: LOGFILE>       Change debugging log file (default: STDERR)
    <log: (?{ CODE })  >     Explicitly add a message to the log


This module adds a small number of new regex constructs that can be used within Perl 5.10 patterns to implement complete recursive-descent parsing.

Perl 5.10 already supports recursive=descent matching, via the new (?<name>...) and (?&name) constructs. For example, here is a simple matcher for a subset of the LaTeX markup language:

    $matcher = qr{

            (?<File>     (?&Element)* )

            (?<Element>  \s* (?&Command)
                      |  \s* (?&Literal)

            (?<Command>  \\ \s* (?&Literal) \s* (?&Options)? \s* (?&Args)? )

            (?<Options>  \[ \s* (?:(?&Option) (?:\s*,\s* (?&Option) )*)? \s* \])

            (?<Args>     \{ \s* (?&Element)* \s* \}  )

            (?<Option>   \s* [^][\$&%#_{}~^\s,]+     )

            (?<Literal>  \s* [^][\$&%#_{}~^\s]+      )

This technique makes it possible to use regexes to recognize complex, hierarchical--and even recursive--textual structures. The problem is that Perl 5.10 doesn't provide any support for extracting that hierarchical data into nested data structures. In other words, using Perl 5.10 you can match complex data, but not parse it into an internally useful form.

An additional problem when using Perl 5.10 regexes to match complex data formats is that you have to make sure you remember to insert whitespace-matching constructs (such as \s*) at every possible position where the data might contain ignorable whitespace. This reduces the readability of such patterns, and increases the chance of errors (typically caused by overlooking a location where whitespace might appear).

The Regexp::Grammars module solves both those problems.

If you import the module into a particular lexical scope, it preprocesses any regex in that scope, so as to implement a number of extensions to the standard Perl 5.10 regex syntax. These extensions simplify the task of defining and calling subrules within a grammar, and allow those subrule calls to capture and retain the components of they match in a proper hierarchical manner.

For example, the above LaTeX matcher could be converted to a full LaTeX parser (and considerably tidied up at the same time), like so:

    use Regexp::Grammars;
    $parser = qr{

        <rule: File>       <[Element]>*

        <rule: Element>    <Command> | <Literal>

        <rule: Command>    \\  <Literal>  <Options>?  <Args>?

        <rule: Options>    \[  <[Option]> ** (,)  \]
        <rule: Args>       \{  <[Element]>*  \}

        <rule: Option>     [^][\$&%#_{}~^\s,]+

        <rule: Literal>    [^][\$&%#_{}~^\s]+

Note that there is no need to explicitly place \s* subpatterns throughout the rules; that is taken care of automatically.

If the Regexp::Grammars version of this regex were successfully matched against some appropriate LaTeX document, each rule would call the subrules specified within it, and then return a hash containing whatever result each of those subrules returned, with each result indexed by the subrule's name.

That is, if the rule named Command were invoked, it would first try to match a backslash, then it would call the three subrules <Literal>, <Options>, and <Args> (in that sequence). If they all matched successfully, the Command rule would then return a hash with three keys: 'Literal', 'Options', and 'Args'. The value for each of those hash entries would be whatever result-hash the subrules themselves had returned when matched.

In this way, each level of the hierarchical regex can generate hashes recording everything its own subrules matched, so when the entire pattern matches, it produces a tree of nested hashes that represent the structured data the pattern matched.

For example, if the previous regex grammar were matched against a string containing:

    \author{D. Conway}

it would automatically extract the following data structure:

        'file' => {
            'element' => [
                    'command' => {
                        'literal' => 'documentclass',
                        'options' => {
                            'option'  => [ 'a4paper', '11pt' ],
                        'args'    => {
                            'element' => [ 'article' ],
                    'command' => {
                        'literal' => 'author',
                        'args' => {
                            'element' => [
                                    'literal' => 'D.',
                                    'literal' => 'Conway',

The data structure that Regexp::Grammars produces from a regex match is available to the surrounding program in the magic variable %/.

Regexp::Grammars provides many features that simplify the extraction of hierarchical data via a regex match, and also some features that can simplify the processing of that data once it has been extracted. The following sections explain each of those features, and some of the parsing techniques they support.

Setting up the module

Just add:

    use Regexp::Grammars;

to any lexical scope. Any regexes within that scope will automatically now implement the new parsing constructs:

    use Regexp::Grammars;

    my $parser = qr/ regex with $extra <chocolatey> grammar bits /x;

Note that you will need to use the /x modifier when declaring a regex grammar. Otherwise, the default "a whitespace character matches exactly that whitespace character" behaviour of Perl regexes will mess up your grammar's parsing.

Once the grammar has been processed, you can then match text against the extended regexes, in the usual manner (i.e. via a =~ match):

    if ($input_text =~ $parser) {

After a successful match, the variable %/ will contain a series of nested hashes representing the structured hierarchical data captured during the parse.

Structure of a Regexp::Grammars grammar

A Regexp::Grammars specification consists of a pattern (which may include both standard Perl 5.10 regex syntax, as well as special Regexp::Grammars directives), followed by one or more rule or token definitions.

For example:

    use Regexp::Grammars;
    my $balanced_brackets = qr{

        # Pattern...
        <paren_pair> | <brace_pair>

        # Rule definition...
        <rule: paren_pair> 
            \(  (?: <escape> | <paren_pair> | <brace_pair> | [^()] )*  \)

        # Rule definition...
        <rule: brace_pair>
            \{  (?: <escape> | <paren_pair> | <brace_pair> | [^{}] )*  \}

        # Token definition...
        <token: escape>
            \\ . 

The initial pattern acts like the "top" rule of the grammar, and must be matched completely for the grammar to match.

The rules and tokens are declarations only and they are not directly matched. Instead, they act like subroutines, and are invoked by name from the initial pattern (or from within a rule or token).

Each rule or token extends from the directive that introduces it up to either the next rule or token directive, or (in the case of the final rule or token) to the end of the grammar.

Tokens vs rules (whitespace handling)

The difference between a token and a rule is that a token treats any whitespace within it exactly as a normal Perl regular expression would. That is, a sequence of whitespace in a token is ignored if the /x modifier is in effect, or else matches the same literal sequence of whitespace characters (if /x is not in effect).

In a rule, any sequence of whitespace (except those at the very start and the very end of the rule) is treated as matching the implicit subrule <.ws>, which is automatically predefined to match optional whitespace (i.e. \s*).

You can explicitly define a <ws> token to change that default behaviour. For example, you could alter the definition of "whitespace" to include Perlish comments, by adding an explicit <token: ws>:

    <token: ws> 
        (?: \s+ | #[^\n]* )*

But be careful not to define <ws> as a rule, as this will lead to all kinds of infinitely recursive unpleasantness.

Calling subrules

To invoke a rule to match at any point, just enclose the rule's name in angle brackets (like in Perl 6). There must be no space between the opening bracket and the rulename. For example::

        file:             # Match literal sequence 'f' 'i' 'l' 'e' ':'
        <name>            # Call <rule: name>
        <options>?        # Call <rule: options> (it's okay if it fails)

        <rule: name>
            # etc.

If you need to match a literal pattern that would otherwise look like a subrule call, just backslash-escape the leading angle:

        file:             # Match literal sequence 'f' 'i' 'l' 'e' ':'
        \<name>           # Match literal sequence '<' 'n' 'a' 'm' 'e' '>' 
        <options>?        # Call <rule: options> (it's okay if it fails)

        <rule: name>
            # etc.

Subrule results

If a subrule call successfully matches, the result of that match is a reference to a hash. That hash reference is stored in the current rule's own result-hash, under the name of the subrule that was invoked. The hash will, in turn, contain the results of any more deeply nested subrule calls, each stored under the name by which the nested subrule was invoked.

In other words, if the rule sentence is defined:

    <rule: sentence>
        <noun> <verb> <object>

then successfully calling the rule:


causes a new hash entry at the current nesting level. That entry's key will be 'sentence' and its value will be a reference to a hash, which in turn will have keys: 'noun', 'verb', and 'object'.

In addition each result-hash has one extra key: the empty string. The value for this key is whatever string the entire subrule call matched. So, for example, a successful call to <sentence> might add something like the following to the current result-hash:

    sentence => {
        ""     => 'I saw a dog',
        noun   => 'I',
        verb   => 'saw',
        object => {
            ""      => 'a dog',
            article => 'a',
            noun    => 'dog',

Note, however, that if the result-hash at any level contains only the empty-string key (i.e. the subrule did not call any sub-subrules or save any of their nested result-hashes), then the hash is "unpacked" and just the matched substring itself if returned.

For example, if <rule: sentence> had been defined:

    <rule: sentence>
        I see dead people

then a successful call to the rule would only add:

    sentence => 'I see dead people'

to the current result-hash.

This is a useful feature because it prevents a series of nested subrule calls from producing very unwieldy data structures. For example, without this automatic unpacking, even the simple earlier example:

    <rule: sentence>
        <noun> <verb> <object>

would produce something needlessly complex, such as:

    sentence => {
        ""     => 'I saw a dog',
        noun   => {
            "" => 'I',
        verb   => {
            "" => 'saw',
        object => {
            ""      => 'a dog',
            article => {
                "" => 'a',
            noun    => {
                "" => 'dog',

Renaming subrule results

It is not always convenient to have subrule results stored under the same name as the rule itself. Rule names should be optimized for understanding the behaviour of the parser, whereas result names should be optimized for understanding the structure of the data. Often those two goals are identical, but not always; sometimes rule names need to describe what the data looks like, while result names need to describe what the data means.

For example, sometimes you need to call the same rule twice, to match two syntactically identical components whose positions give then semantically distinct meanings:

    <rule: copy_cmd>
        copy <file> <file>

The problem here is that, if the second call to <file> succeeds, its result-hash will be stored under the key 'file', clobbering the data that was returned from the first call to <file>.

To avoid such problems, Regexp::Grammars allows you to alias any subrule call, so that it is still invoked by the original name, but its result-hash is stored under a different key. The syntax for that is: <alias=rulename>. For example:

    <rule: copy_cmd>
        copy <from=file> <to=file>

Here, <rule: file> is called twice, with the first result-hash being stored under the key 'from', and the second result-hash being stored under the key 'to'.

Note, however, that the alias before the = must be a proper identifier (i.e. a letter or underscore, followed by letters, digits, and/or underscores). Aliases that start with an underscore and aliases named MATCH have special meaning (see "Private subrule calls" and "Result distillation" respectively).

Aliases can also be useful for normalizing data that may appear in different formats and sequences. For example:

    <rule: copy_cmd>
        copy <from=file>        <to=file>
      | dup    <to=file>  as  <from=file>
      |      <from=file>  ->    <to=file>
      |        <to=file>  <-  <from=file>

Here, regardless of which order the old and new files are specified, the result-hash always gets:

    copy_cmd => {
        from => 'oldfile',
          to => 'newfile',

List-like subrule calls

If a subrule call is quantified with a repetition specifier:

    <rule: file_sequence>

then each repeated match overwrites the corresponding entry in the surrounding rule's result-hash, so only the result of the final repetition will be retained. That is, if the above example matched the string "foo.pl bar.py baz.php", then the result-hash would contain:

    file_sequence {
        ""   => 'foo.pl bar.py baz.php',
        file => 'baz.php',

Usually, that's not the desired outcome, so Regexp::Grammars provides another mechanism by which to call a subrule; one that saves all repetitions of its results.

A regular subrule call consists of the rule's name surrounded by angle brackets. If, instead, you surround the rule's name with <[...]> (angle and square brackets) like so:

    <rule: file_sequence>

then the rule is invoked in exactly the same way, but the result of that submatch is pushed onto an array nested inside the appropriate result-hash entry. In other words, if the above example matched the same "foo.pl bar.py baz.php" string, the result-hash would contain:

    file_sequence {
        ""   => 'foo.pl bar.py baz.php',
        file => [ 'foo.pl', 'bar.py', 'baz.php' ],

This "listifying subrule call" can also be useful for non-repeated subrule calls, if the same subrule is invoked in several places in a grammar. For example if a cmdline option could be given either one or two values, you might parse it:

    <rule: size_option>   
        -size <[size]> (?: x <[size]> )?

The result-hash entry for 'size' would then always contain an array, with either one or two elements, depending on the input being parsed.

Listifying subrules can also be given aliases, just like ordinary subrules. The alias is always specified inside the square brackets:

    <rule: size_option>   
        -size <[size=pos_integer]> (?: x <[size=pos_integer]> )?

Here, the sizes are parsed using the pos_integer rule, but saved in the result-hash in an array under the key 'size'.


Aliases can also be given to standard Perl subpatterns, as well as to code blocks within a regex. The syntax for subpatterns is:


In other words, the syntax is exactly like an aliased subrule call, except that the rule name is replaced with a set of parentheses containing the subpattern. Any parentheses--capturing or non-capturing--will do.

The effect of aliasing a standard subpattern is to cause whatever that subpattern matches to be saved in the result-hash, using the alias as its key. For example:

    <rule: file_command>

        <cmd=(mv|cp|ln)>  <from=file>  <to=file>

Here, the <cmd=(mv|cp|ln)> is treated exactly like a regular (mv|cp|ln), but whatever substring it matches is saved in the result-hash under the key 'cmd'.

The syntax for aliasing code blocks is:

    <ALIAS= (?{ your($code->here) }) >

Note, however, that the code block must be specified in the standard Perl 5.10 regex notation: (?{...}). A common mistake is to write:

    <ALIAS= { your($code->here } >

instead, which will attempt to interpolate $code before the regex is even compiled, as such variables are only "protected" from interpolation inside a (?{...}).

When correctly specified, this construct executes the code in the block and saves the result of that execution in the result-hash, using the alias as its key. Aliased code blocks are useful for adding semantic information based on which branch of a rule is executed. For example, consider the copy_cmd alternatives shown earlier:

    <rule: copy_cmd>
        copy <from=file>        <to=file>
      | dup    <to=file>  as  <from=file>
      |      <from=file>  ->    <to=file>
      |        <to=file>  <-  <from=file>

Using aliased code blocks, you could add an extra field to the result- hash to describe which form of the command was detected, like so:

    <rule: copy_cmd>
        copy <from=file>        <to=file>  <type=(?{ 'std' })> 
      | dup    <to=file>  as  <from=file>  <type=(?{ 'rev' })> 
      |      <from=file>  ->    <to=file>  <type=(?{ 'fwd' })> 
      |        <to=file>  <-  <from=file>  <type=(?{ 'bwd' })> 

Now, if the rule matched, the result-hash would contain something like:

    copy_cmd => {
        from => 'oldfile',
          to => 'newfile',
        type => 'fwd',

Note that, in addition to the semantics described above, aliased subpatterns and code blocks also become visible to Regexp::Grammars' integrated debugger (see Debugging).

Amnesiac subrule calls

By default, every subrule call saves its result into the result-hash, either under its own name, or under an alias.

However, sometimes you may want to refactor some literal part of a rule into one or more subrules, without having those submatches added to the result-hash. The syntax for calling a subrule, but ignoring its return value is:


(which is stolen directly from Perl 6).

For example, you may prefer to rewrite a rule such as:

    <rule: paren_pair> 

            (?: <escape> | <paren_pair> | <brace_pair> | [^()] )*

without any literal matching, like so:

    <rule: paren_pair> 

            (?: <escape> | <paren_pair> | <brace_pair> | <.non_paren> )*
    <token: left_paren>   \(
    <token: right_paren>  \)
    <token: non_paren>    [^()]

Moreover, as the individual components inside the parentheses probably aren't being captured for any useful purpose either, you could further optimize that to:

    <rule: paren_pair> 

            (?: <.escape> | <.paren_pair> | <.brace_pair> | <.non_paren> )*

Note that you can also use the dot modifier on an aliased subpattern:

    <.Alias= (SUBPATTERN) >

This seemingly contradictory behaviour (of giving a subpattern a name, then deliberately ignoring that name) actually does make sense in one situation. Providing the alias makes the subpattern visible to the debugger, while using the dot stops it from affecting the result-hash. See "Debugging non-grammars" for an example of this usage.

Private subrule calls

If a rule name (or an alias) begins with an underscore:

     <_RULENAME>       <_ALIAS=RULENAME>  

then matching proceeds as normal, and any result that is returned is stored in the current result-hash in the usual way.

However, when any rule finishes (and just before it returns) it first filters its result-hash, removing any entries whose keys begin with an underscore. This means that any subrule with an underscored name (or with an underscored alias) remembers its result, but only until the end of the current rule. Its results are effectively private to the current rule.

This is especially useful in conjunction with result distillation.

Matching separated lists

One of the commonest tasks in text parsing is to match a list of unspecified length, in which items are separated by a fixed token. Things like:

    1, 2, 3 , 4 ,13, 91        # Numbers separated by commas and spaces

    g-c-a-g-t-t-a-c-a          # Bases separated by dashes

    /usr/local/bin             # Names separated by directory markers

    /usr:/usr/local:bin        # Directories separated by colons

The usual construct required to parse these kinds of structures is either:

    <rule: list>

        <item> <separator> <list               # recursive definition
      | <item>                                 # base case

Or, more efficiently, but less prettily:

    <rule: list>

        <[item]> (?: <separator> <[item]> )*   # iterative definition

Because this is such a common requirement, Regexp::Grammars provides a cleaner way to specify the iterative version. The syntax is taken from Perl 6:

    <rule: list>

        <[item]> ** <separator>                # iterative definition

This is a repetition specifier on the first subrule (hence the use of ** as the marker, to reflect the repetitive behaviour of *). However, the number of repetitions is controlled by the second subrule: the first subrule will be repeatedly matched for as long as the second subrule matches immediately after it.

So, for example, you can match a sequence of numbers separated by commas with:

    <[number]> ** <comma>

    <token: number>  \d+
    <token: comma>   \s* , \s*

Note that it's important to use the <[...]> form for the items being matched, so that all of them are saved in the result hash. You can also save all the separators (if that's important):

    <[number]> ** <[comma]>

The repeated item must be specified as a subrule call fo some kind, but the separators may be specified either as a subrule or a bracketed pattern. For example:

    <[number]> ** ( , )

The separator must always be specified in matched delimiters of some kind: either matching <...> or matching (...). A common error is to write:

    <[number]> ** ,

You can also use a pattern as the item matcher, but it must be aliased into a subrule:

    <[item=(\d+)]> ** ( , )

Matching hash keys

In some situations a grammar may need a rule that matches dozens, hundreds, or even thousands of one-word alternatives. For example, when matching command names, or valid userids, or English words. In such cases it is often impractical (and always inefficient) to list all the alternatives between | alterators:

    <rule: shell_cmd>
        a2p | ac | apply | ar | automake | awk | ...
        # ...and 400 lines later
        ... | zdiff | zgrep | zip | zmore | zsh

    <rule: valid_word>
        a | aa | aal | aalii | aam | aardvark | aardwolf | aba | ...
        # ...and 40,000 lines later... 
        ... | zymotize | zymotoxic | zymurgy | zythem | zythum

To simplify such cases, Regexp::Grammars provides a special construct that allows you to specify all the alternatives as the keys of a normal hash. The syntax for that construct is simply to put the hash name inside angle brackets (with no space between the angles and the hash name).

Which means that the rules in the previous example could also be written:

    <rule: shell_cmd>

    <rule: valid_word>

provided that the two hashes (%cmds and %dict) are visible in the scope where the grammar is created.

Internally, the construct is converted to something equivalent to:

    <rule: shell_cmd>
        (<.hk>)  <require: exists $cmds{$CAPTURE}>

    <rule: valid_word>
        (<.hk>)  <require: exists $dict{$CAPTURE}>

The special <hk> rule is created automatically, and defaults to \S+, but you can also define it explicitly to handle other kinds of keys. For example:

    <rule: hk>
        .+            # Key may be any number of chars on a single line

    <rule: hk>
        [ACGT]{10,}   # Key is a base sequence of at least 10 pairs

Matching a hash key in this way is typically significantly faster than matching a full set of alternations. Specifically, it is O(length of longest potential key), instead of O(number of keys).

Common parsing techniques

Result distillation

Normally, calls to subrules produce nested result-hashes within the current result-hash. Those nested hashes always have at least one automatically supplied key (""), whose value is the entire substring that the subrule matched.

If there are no other nested captures within the subrule, there will be no other keys in the result-hash. This would be annoying as a typical nested grammar would then produce results consisting of hashes of hashes, with each nested hash having only a single key (""). This in turn would make postprocessing the result-hash (in %/) far more complicated than it needs to be.

To avoid this behaviour, if a subrule's result-hash doesn't contain any keys except "", the module "flattens" the result-hash, by replacing it with the value of its single key.

So, for example, the grammar:

    mv \s* <from> \s* <to>

    <rule: from>   [\w/.-]+
    <rule: to>     [\w/.-]+

doesn't return a result-hash like this:

        ""     => 'mv /usr/local/lib/libhuh.dylib  /dev/null/badlib',
        'from' => { "" => '/usr/local/lib/libhuh.dylib' },
        'to'   => { "" => '/dev/null/badlib'            },

Instead, it returns:

        ""     => 'mv /usr/local/lib/libhuh.dylib  /dev/null/badlib',
        'from' => '/usr/local/lib/libhuh.dylib',
        'to'   => '/dev/null/badlib',

That is, because the 'from' and 'to' subhashes each have only a single entry, they are each "flattened" to the value of that entry.

This flattening also occurs if a result-hash contains only "private" keys (i.e. keys starting with underscores). For example:

    mv \s* <from> \s* <to>

    <rule: from>   <_dir=path>? <_file=filename>
    <rule: to>     <_dir=path>? <_file=filename>
    <token: path>      [\w/.-]*/
    <token: filename>  [\w.-]+

Here, the from rule produces a result like this:

    from => {
          "" => '/usr/local/bin/perl',
        _dir => '/usr/local/bin/',
       _file => 'perl',

which is automatically stripped of "private" keys, leaving:

    from => {
          "" => '/usr/local/bin/perl',

which is then automatically flattened to:

    from => '/usr/local/bin/perl'

Manual result distillation

Regexp::Grammars also offers full manual control over the distillation process. If you use the reserved word MATCH as the alias for a subrule call:


or a subpattern match:

    <MATCH=( \w+ )>

or a code block:

    <MATCH=(?{ 42 })>

then the current rule will treat the return value of that subrule, pattern, or code block as its complete result, and return that value instead of the usual result-hash it constructs. This is the case even if the result has other entries that would normally also be returned.

For example, in a rule like:

    <rule: term>
        | <left_paren> <MATCH=expr> <right_paren>

The use of MATCH aliases causes the rule to return either whatever <literal> returns, or whatever <expr> returns (provided it's between left and right parentheses).

Note that, in this second case, even though <left_paren> and <right_paren> are captured to the result-hash, they are not returned, because the MATCH alias overrides the normal "return the result-hash" semantics and returns only what its associated subrule (i.e. <expr>) produces.

Programmatic result distillation

It's also possible to control what a rule returns from within a code block. Regexp::Grammars provides a set of reserved variables that give direct access to the result-hash.

The result-hash itself can be accessed as %MATCH within any code block inside a rule. For example:

    <rule: sum> 
        <X=product> \+ <Y=product>
            <MATCH=(?{ $MATCH{X} + $MATCH{Y} })>

Here, the rule matches a product (aliased 'X' in the result-hash), then a literal '+', then another product (aliased to 'Y' in the result-hash). The rule then executes the code block, which accesses the two saved values (as $MATCH{X} and $MATCH{Y}), adding them together. Because the block is itself aliased to MATCH, the sum produced by the block becomes the (only) result of the rule.

It is also possible to set the rule result from within a code block (instead of aliasing it). The special "override" return value is represented by the special variable $MATCH. So the previous example could be rewritten:

    <rule: sum> 
        <X=product> \+ <Y=product>
            (?{ $MATCH = $MATCH{X} + $MATCH{Y} })

Both forms are identical in effect. Any assignment to $MATCH overrides the normal "return all subrule results" behaviour.

Assigning to $MATCH directly is particularly handy if the result may not always be "distillable", for example:

    <rule: sum> 
        <X=product> \+ <Y=product>
            (?{ if (!ref $MATCH{X} && !ref $MATCH{Y}) {
                    # Reduce to sum, if both terms are simple scalars...
                    $MATCH = $MATCH{X} + $MATCH{Y};
                else {
                    # Return full syntax tree for non-simple case...
                    $MATCH{op} = '+';

Note that you can also partially override the subrule return behaviour. Normally, the subrule returns the complete text it matched under the "empty key" of its result-hash. That is, of course, $MATCH{""}, so you can override just that behaviour by directly assigning to that entry.

For example, if you have a rule that matches key/value pairs from a configuration file, you might prefer that any trailing comments not be included in the "matched text" entry of the rule's result-hash. You could hide such comments like so:

    <rule: config_line>
        <key> : <value>  <comment>?
                # Edit trailing comments out of "matched text" entry...
                $MATCH = "$MATCH{key} : $MATCH{value}";

Some more examples of the uses of $MATCH:

    <rule: FuncDecl>
      # Keyword  Name               Keep return the name (as a string)...
        func     <Identifier> ;     (?{ $MATCH = $MATCH{'Identifier'} })

    <rule: NumList>
      # Numbers in square brackets...
            ( \d+ (?: , \d+)* )

      # Return only the numbers...
        (?{ $MATCH = $CAPTURE })

    <token: Cmd>
      # Match standard variants then standardize the keyword...
        (?: mv | move | rename )      (?{ $MATCH = 'mv'; })

Parse-time data processing

Using code blocks in rules, it's often possible to fully process data as you parse it. For example, the <sum> rule shown in the previous section might be part of a simple calculator, implemented entirely in a single grammar. Such a calculator might look like this:

    my $calculator = do{
        use Regexp::Grammars;

            <rule: Answer>
                <X=Mult> \+ <Y=Answer>
                    <MATCH= (?{ $MATCH{X} + $MATCH{Y} })>
              | <X=Mult> - <Y=Answer>
                    <MATCH= (?{ $MATCH{X} - $MATCH{Y} })>

            <rule: Mult>
                <X=Pow> \* <Y=Mult>
                    <MATCH= (?{ $MATCH{X} * $MATCH{Y} })>
              | <X=Pow>  / <Y=Mult>
                    <MATCH= (?{ $MATCH{X} / $MATCH{Y} })>
              | <X=Pow>  % <Y=Mult>
                    <MATCH= (?{ $MATCH{X} % $MATCH{Y} })>

            <rule: Pow>
                <X=Term> \^ <Y=Pow>
                    <MATCH= (?{ $MATCH{X} ** $MATCH{Y}; })>

            <rule: Term>
              | \(  <MATCH=Answer>  \)

            <token: Literal>
                    <MATCH= ( [+-]? \d++ (?: \. \d++ )?+ )>

    while (my $input = <>) {
        if ($input =~ $calculator) {
            say "--> $/{Answer}";

Because every rule computes a value using the results of the subrules below it, and aliases that result to its MATCH, each rule returns a complete evaluation of the subexpression it matches, passing that back to higher-level rules, which then do the same.

Hence, the result returned to the very top-level rule (i.e. to <Answer>) is the complete evaluation of the entire expression that was matched. That means that, in the very process of having matched a valid expression, the calculator has also computed the value of that expression, which can then simply be printed directly.

It is often possible to have a grammar fully (or sometimes at least partially) evaluate or transform the data it is parsing, and this usually leads to very efficient and easy-to-maintain implementations.

The main limitation of this technique is that the data has to be in a well-structured form, where subsets of the data can be evaluated using only local information. In cases where the meaning of the data is distributed through that data non-hierarchically, or relies on global state, or on external information, it is often better to have the grammar simply construct a complete syntax tree for the data first, and then evaluate that syntax tree separately, after parsing is complete. The following section describes a feature of Regexp::Grammars that can make this second style of data processing simpler and more maintainable.

Object-oriented parsing

When a grammar has parsed successfully, the %/ variable will contain a series of nested hashes (and possibly arrays) representing the hierarchical structure of the parsed data.

Typically, the next step is to walk that tree, extracting or converting or otherwise processing that information. If the tree has nodes of many different types, it can be difficult to build a recursive subroutine that can navigate it easily.

A much cleaner solution is possible if the nodes of the tree are proper objects. In that case, you just define a process() or traverse() method for eah of the classes, and have every node call that method on each of its children. For example, if the parser were to return a tree of nodes representing the contents of a LaTeX file, then you could define the following methods:

    sub Latex::file::explain
        my ($self, $level) = @_;
        for my $element (@{$self->{element}}) {

    sub Latex::element::explain {
        my ($self, $level) = @_;
        (  $self->{command} || $self->{literal})->explain($level)

    sub Latex::command::explain {
        my ($self, $level) = @_;
        say "\t"x$level, "Command:";
        say "\t"x($level+1), "Name: $self->{name}";
        if ($self->{options}) {
            say "\t"x$level, "\tOptions:";
        for my $arg (@{$self->{arg}}) {
            say "\t"x$level, "\tArg:";

    sub Latex::options::explain {
        my ($self, $level) = @_;
        $_->explain($level) foreach @{$self->{option}};

    sub Latex::literal::explain {
        my ($self, $level, $label) = @_;
        $label //= 'Literal';
        say "\t"x$level, "$label: ", $self->{q{}};

and then simply write:

    if ($text =~ $LaTeX_parser) {

and the chain of explain() calls would cascade down the nodes of the tree, each one invoking the appropriate explain() method according to the type of node encountered.

The only problem is that, by default, Regexp::Grammars returns a tree of plain-old hashes, not LaTeX::whatever objects. Fortunately, it's easy to request that the result hashes be automatically blessed into the appropriate classes, using the <objrule:...> and <objtoken:...> directives.

These directives are identical to the <rule:...> and <token:...> directives (respectively), except that the rule or token they create will also bless the hash it normally returns, converting it to an object of a class whose name is the same as the rule or token itself.

For example:

    <objrule: Element>
        # ...Defines a rule that can be called as <Element>
        # ...and which returns a hash-based Element object

The IDENTIFIER of the rule or token may also be fully qualified. In such cases, the rule or token is defined using only the final "short name", but the result object is blessed using the fully qualified "long name". For example:

    <objrule: LaTeX::Element> 
        # ...Defines a rule that can be called as <Element>
        # ...and which returns a hash-based LaTeX::Element object

This can be useful to ensure that returned objects don't collide with other namespaces in your program.

Note that you can freely mix object-returning and plain-old-hash-returning rules and tokens within a single grammar, though you have to be careful not to subsequently try to call a method on any of the unblessed nodes.


Regexp::Grammars provides a number of features specifically designed to help debug both grammars and the data they parse.

All debugging messages are written to a log file (which, by default, is just STDERR). However, you can specify a disk file explicitly by placing a <logfile:...> directive at the start of your grammar:

    $grammar = qr{

        <logfile: LaTeX_parser_log >

        \A <LaTeX_file> \Z    # Pattern to match

        <rule: LaTeX_file>
            # etc.

You can also explicitly specify that messages go to the terminal:

        <logfile: - >

Debugging grammar creation with <logfile:...>

Whenever a log file has been directly specified, Regexp::Grammars automatically does verbose static analysis of your grammar. That is, whenever it compiles a grammar containing an explicit <logfile:...> directive it logs a series of messages explaining how it has interpreted the various components of that grammar. For example, the following grammar:

    <logfile: parser_log > 


    <rule: cmd>
        mv <from=file> <to=file>
      | cp <source> <[file]>  <.comment>?

would produce the following analysis in the 'parser_log' file:

    info | Processing the main regex before any rule definitions
         |    |
         |    |...Treating <cmd> as:
         |    |      |  match the subrule <cmd> 
         |    |       \ saving the match in $MATCH{'cmd'}
         |    |
         |     \___End of main regex
    info | Defining a rule: <cmd>
         |    |...Returns: a hash
         |    |
         |    |...Treating ' mv ' as:
         |    |       \ normal Perl regex syntax
         |    |
         |    |...Treating <from=file> as:
         |    |      |  match the subrule <file> 
         |    |       \ saving the match in $MATCH{'from'}
         |    |
         |    |...Treating <to=file> as:
         |    |      |  match the subrule <file> 
         |    |       \ saving the match in $MATCH{'to'}
         |    |
         |    |...Treating ' | cp ' as:
         |    |       \ normal Perl regex syntax
         |    |
         |    |...Treating <source> as:
         |    |      |  match the subrule <source> 
         |    |       \ saving the match in $MATCH{'source'}
         |    |
         |    |...Treating <[file]> as:
         |    |      |  match the subrule <file> 
         |    |       \ appending the match to $MATCH{'file'}
         |    |
         |    |...Treating <.comment>? as:
         |    |      |  match the subrule <comment> if possible
         |    |       \ but don't save anything
         |    |
         |     \___End of rule definition

This kind of static analysis is a useful starting point in debugging a miscreant grammar, because it enables you to see what you actually specified (as opposed to what you thought you'd specified).

Debugging grammar execution with <debug:...>

Regexp::Grammars also provides a simple interactive debugger, with which you can observe the process of parsing and the data being collected in any result-hash.

To initiate debugging, place a <debug:...> directive anywhere in your grammar. When parsing reaches that directive the debugger will be activated, and the command specified in the directive immediately executed. The available commands are:

    <debug: on>    - Enable debugging, stop when entire grammar matches
    <debug: match> - Enable debugging, stope when a rule matches
    <debug: try>   - Enable debugging, stope when a rule is tried
    <debug: off>   - Disable debugging and continue parsing silently

    <debug: continue> - Synonym for <debug: on>
    <debug: run>      - Synonym for <debug: on>
    <debug: step>     - Synonym for <debug: try>

These directives can be placed anywhere within a grammar and take effect when that point is reached in the parsing. Hence, adding a <debug:step> directive is very much like setting a breakpoint at that point in the grammar. Indeed, a common debugging strategy is to turn debugging on and off only around a suspect part of the grammar:

    <rule: tricky>   # This is where we think the problem is...
        <preamble> <text> <postscript>

Once the debugger is active, it steps through the parse, reporting rules that are tried, matches and failures, backtracking and restarts, and the parser's location within both the grammar and the text being matched. That report looks like this:

    ===============> Trying <grammar> from position 0
    > cp file1 file2 |...Trying <cmd>   
                     |   |...Trying <cmd=(cp)>  
                     |   |    \FAIL <cmd=(cp)>
                     |    \FAIL <cmd>
                      \FAIL <grammar>
    ===============> Trying <grammar> from position 1
     cp file1 file2  |...Trying <cmd>   
                     |   |...Trying <cmd=(cp)>  
     file1 file2     |   |    \_____<cmd=(cp)> matched 'cp' 
    file1 file2      |   |...Trying <[file]>+   
     file2           |   |    \_____<[file]>+ matched 'file1'   
                     |   |...Trying <[file]>+   
    [eos]            |   |    \_____<[file]>+ matched ' file2'  
                     |   |...Trying <[file]>+   
                     |   |    \FAIL <[file]>+
                     |   |...Trying <target>    
                     |   |   |...Trying <file>
                     |   |   |    \FAIL <file>
                     |   |    \FAIL <target>
     <~~~~~~~~~~~~~~ |   |...Backtracking 5 chars and trying new match
    file2            |   |...Trying <target>    
                     |   |   |...Trying <file>
                     |   |   |    \____ <file> matched 'file2'
    [eos]            |   |    \_____<target> matched 'file2'    
                     |    \_____<cmd> matched ' cp file1 file2' 
                      \_____<grammar> matched ' cp file1 file2' 

The first column indicates the point in the input at which the parser is trying to match, as well as any backtracking or forward searching it may need to do. The remainder of the columns track the parser's hierarchical traversal of the grammar, indicating which rules are tried, which succeed, and what they match.

Provided the logfile is a terminal (as it is by default), the debugger also pauses at various points in the parsing process--before trying a rule, after a rule succeeds, or at the end of the parse--according to the most recent command issued. When it pauses, you can issue a new command by entering a single letter:

    m       - to continue until the next subrule matches
    t or s  - to continue until the next subrule is tried
    r or c  - to continue to the end of the grammar
    o       - to switch off debugging

Note that these are the first letters of the corresponding <debug:...> commands, listed earlier. Just hitting ENTER while the debugger is paused repeats the previous command.

While the debugger is paused you can also type a 'd', which will display the result-hash for the current rule. This can be useful for detecting which rule isn't returning the data you expected.

User-defined logging with <log:...>

Both static and interactive debugging send a series of predefined log messages to whatever log file you have specified. It is also possible to send additional, user-defined messages to the log, using the <log:...> directive.

This directive expects either a simple text or a codeblock as its single argument. If the argument is a code block, that code is expected to return the text of the message; if the argument is anything else, that something else is the literal message. For example:

    <rule: ListElem>

        <Elem=   ( [a-z]\d+) >
            <log: Checking for a suffix, too...>

        <Suffix= ( : \d+   ) >?
            <log: (?{ "ListElem: $MATCH{Elem} and $MATCH{Suffix}" })>

User-defined log messages implemented using a codeblock can also specify a severity level. If the codeblock of a <log:...> directive returns two or more values, the first is treated as a log message severity indicator, and the remaining values as separate lines of text to be logged. For example:

    <rule: ListElem>
        <Elem=   ( [a-z]\d+) >
        <Suffix= ( : \d+   ) >?

            <log: (?{
                warn => "Elem was: $MATCH{Elem}", 
                        "Suffix was $MATCH{Suffix}",

When they are encountered, user-defined log messages are interspersed between any automatic log messages (i.e. from the debugger), at the correct level of nesting for the current rule.

Debugging non-grammars

It is possible to use Regexp::Grammars without creating any subrule definitions, simply to debug a recalcitrant regex. For example, if the following regex wasn't working as expected:

    my $balanced_brackets = qr{
        \(             # left delim
            \\         # escape or
        |   (?R)       # recurse or
        |   .          # whatever
        \)             # right delim

you could instrument it with aliased subpatterns and then debug it step-by-step, using Regexp::Grammars:

    use Regexp::Grammars;

    my $balanced_brackets = qr{

        <.left_delim=  (  \(  )>
            <.escape=  (  \\  )>
        |   <.recurse= ( (?R) )>
        |   <.whatever=(  .   )>
        <.right_delim= (  \)  )>

    while (<>) {
        say 'matched' if /$balanced_brackets/;

Note the use of amnesiac aliased subpatterns to avoid needlessly building a result-hash. Alternatively, you could use listifying aliases to preserve the matching structure as an additional debugging aid:

    use Regexp::Grammars;

    my $balanced_brackets = qr{

        <[left_delim=  (  \(  )]>
            <[escape=  (  \\  )]>
        |   <[recurse= ( (?R) )]>
        |   <[whatever=(  .   )]>
        <[right_delim= (  \)  )]>

    if ( '(a(bc)d)' =~ /$balanced_brackets/) {
        use Data::Dumper 'Dumper';
        warn Dumper \%/;

Handling errors when parsing

Assuming you have correctly debugged your grammar, the next source of problems will likely be invalid input (especially if that input is being provided interactively). So Regexp::Grammars also provides some support for detecting when a parse is likely to fail...and informing the user why.


The <require:...> directive is useful for testing conditions that it's not easy (or even possible) to check within the syntax of the the regex itself. For example:

    <rule: IPV4_Octet_Decimal>
        # Up three digits...
        <MATCH= ( \d{1,3}+ )>
        # ...but less that 256...
        <require: (?{ $MATCH <= 255 })>

A require expects a regex codeblock as its argument and succeeds if the final value of that codeblock is true. If the final value is false, the directive fails and the rule starts backtracking.

Note, in this example that the digits are matched with \d{1,3}+ . The trailing + prevents the {1,3} repetition from backtracking to a smaller number of digits if the <require:...> fails.

Error messages

The module has limited support for error reporting from within a grammar, in the form of the <error:...> and <warning:...> directives.

The <error: MSG> directive queues a conditional error message within @! and then fails to match (that is, it is equivalent to a (?!) when matching). For example:

    <rule: ListElem>
      | <ClientName>
      | <error: (?{ $errcount++ . ': Missing list element' })>

The error message is conditional in the sense that, if any surrounding rule subsequently matches, the message is automatically removed from @!. This implies that you can queue up as many error messages as you wish, but they will only remain in @! if the match ultimately fails. Moreover, only those error messages originating from rules that actually contributed to the eventual failure-to-match will remain in @!.

If a code block is specified as the argument, the error message is whatever final value is produced when the block is executed. Note that this final value does not have to be a string (though it does have to be a scalar).

    <rule: ListElem>
      | <ClientName>
      | <error: (?{
            # Return a hash, with the error information...
            { errnum => $errcount++, msg => 'Missing list element' }

If anything else is specified as the argument, it is treated as a literal error string (and may not contain an unbalanced '<' or '>', nor any interpolated variables).

However, if the literal error string begins with "Expected " or "Expecting ", then the error string automatically has the following "context suffix" appended:

    , but found '$CONTEXT' instead

For example:

    qr{ <Arithmetic_Expression>                # ...Match arithmetic expression
      |                                        # Or else
        <error: Expected a valid expression>   # ...Report error, and fail

        # Rule definitions here...

On an invalid input this example might produce an error message like:

    "Expected a valid expression, but found '(2+3]*7/' instead"

The value of the special $CONTEXT variable is found by looking ahead in the string being matched against, to locate the next sequence of non-blank characters after the current parsing position. This variable may also be explicitly used within the <error: (?{...})> form of the directive.

As a special case, if you omit the message entirely from the directive, it is supplied automatically, derived from the name of the current rule. For example, if the following rule were to fail to match:

    <rule: Arithmetic_expression>
          <Multiplicative_Expression> ** ([+-])
        | <error:>

the error message queued would be:

    "Expected arithmetic expression, but found 'one plus two' instead"

Note however, that it is still essential to include the colon in the directive. A common mistake is to write:

    <rule: Arithmetic_expression>
          <Multiplicative_Expression> ** ([+-])
        | <error>

which merely attempts to call <rule: error> if the first alternative fails.

Warning messages

Sometimes, you want to detect problems, but not invalidate the entire parse as a result. For those occasions, the module provides a "less stringent" form of error reporting: the <warning:...> directive.

This directive is exactly the same as an <error:...> in every respect except that it does not induce a failure to match at the point it appears.

The directive is, therefore, useful for reporting non-fatal problems in a parse. For example:

    qr{ \A            # ...Match only at start of input
        <ArithExpr>   # ...Match a valid arithmetic expression

            # Should be at end of input...
            \s* \Z                            
            # If not, report the fact but don't fail...
            <warning: Expected end-of-input> 
            <warning: (?{ "Extra junk at index $INDEX: $CONTEXT" })>

        # Rule definitions here...

Note that, because they do not induce failure, two or more <warning:...> directives can be "stacked" in sequence, as in the previous example.

Scoping considerations

If you intend to use a grammar as part of a larger program that contains other (non-grammatical) regexes, it is more efficient--and less error-prone--to avoid having Regexp::Grammars process those regexes as well. So it's often a good idea to declare your grammar in a do block, thereby restricting the scope of the module's effects.

For example:

    my $grammar = do {
        use Regexp::Grammars;

            <rule: file>

            <rule: prelude>
                # etc.

Because the effects of Regexp::Grammars are lexically scoped, any regexes defined outside that do block will be unaffected by the module.


Perl API

use Regexp::Grammars;

Causes all regexes in the current lexical scope to be compile-time processed for grammar elements.

$str =~ $grammar
$str =~ /$grammar/

Attempt to match the grammar against the string, building a nested data structure from it.


This hash is assigned the nested data structure created by any successful match of a grammar regex.


This array is assigned the queue of error messages created by any unsuccessful match attempt of a grammar regex.

Grammar syntax



Define a rule whose name is specified by the supplied identifier.

Everything following the <rule:...> directive (up to the next <rule:...> or <token:...> directive) is treated as part of the rule being defined.

Any whitespace in the rule is replaced by a call to the <.ws> subrule (which defaults to matching \s*, but may be explicitly redefined).


Define a rule whose name is specified by the supplied identifier.

Everything following the <token:...> directive (up to the next <rule:...> or <token:...> directive) is treated as part of the rule being defined.

Any whitespace in the rule is ignored (under the /x modifier), or explicitly matched (if /x is not used).

<objrule: IDENTIFIER>
<objtoken: IDENTIFIER>

Identical to a <rule: IDENTIFIER> or <token: IDENTIFIER> declaration, except that the rule or token will also bless the hash it normally returns, converting it to an object of a class whose name is the same as the rule or token itself.

<require: (?{ CODE }) >

The code block is executed and if its final value is true, matching continues from the same position. If the block's final value is false, the match fails at that point and starts backtracking.

<error: (?{ CODE }) >
<error: LITERAL TEXT >
<error: >

This directive queues a conditional error message within @! and then fails to match (that is, it is equivalent to a (?!) when matching).

<warning: (?{ CODE }) >
<warning: LITERAL TEXT >

This directive is exactly the same as an <error:...> in every respect except that it does not induce a failure to match at the point it appears. That is, it is equivalent to a (?=) ["succeed and continue matching"], rather than a (?!) ["fail and backtrack"].

<debug: COMMAND >

During the matching of grammar regexes send debugging and warning information to the specified log file (see <logfile: LOGFILE>).

The available COMMAND's are:

    <debug: continue>    ___ Debug until end of complete parse
    <debug: run>         _/

    <debug: on>          ___ Debug until next subrule match
    <debug: match>       _/

    <debug: try>         ___ Debug until next subrule call or match
    <debug: step>        _/

    <debug: off>         ___ No debugging

See also the $DEBUG special variable.

<logfile: LOGFILE>
<logfile: - >

During the compilation of grammar regexes, send debugging and warning information to the specified LOGFILE (or to *STDERR if - is specified).

If the specified LOGFILE name contains a %t, it is replaced with a (sortable) "YYYYMMDD.HHMMSS" timestamp. For example:

    <logfile: test-run-%t >

executed at around 9.30pm on the 21st of March 2009, would generate a log file named: test-run-20090321.213056

<log: (?{ CODE }) >

Append a message to the log file. If the argument is a code block, that code is expected to return the text of the message; if the argument is anything else, that something else is the literal message.

If the block returns two or more values, the first is treated as a log message severity indicator, and the remaining values as separate lines of text to be logged.

Subrule calls


Call the subrule whose name is IDENTIFIER.

If it matches successfully, save the hash it returns in the current scope's result-hash, under the key 'IDENTIFIER'.


Call the subrule whose name is IDENTIFIER_1.

If it matches successfully, save the hash it returns in the current scope's result-hash, under the key 'IDENTIFIER_2'.

In other words, the IDENTIFIER_1= prefix changes the key under which the result of calling a subrule is stored.


Call the subrule whose name is IDENTIFIER. Don't save the hash it returns.

In other words, the "dot" prefix disables saving of subrule results.


Match the subpattern PATTERN.

If it matches successfully, capture the substring it matched and save that substring in the current scope's result-hash, under the key 'IDENTIFIER'.


Match the subpattern PATTERN. Don't save the substring it matched.


Match a sequence of non-whitespace then verify that the sequence is a key in the specified hash

If it matches successfully, capture the sequence it matched and save that substring in the current scope's result-hash, under the key 'IDENTIFIER'.


Match a key from the hash. Don't save the substring it matched.


Execute the specified CODE.

Save the result (of the final expression that the CODE evaluates) in the current scope's result-hash, under the key 'IDENTIFIER'.


Call the subrule whose name is IDENTIFIER.

If it matches successfully, append the hash it returns to a nested array within the current scope's result-hash, under the key <'IDENTIFIER'>.


Call the subrule whose name is IDENTIFIER_1.

If it matches successfully, append the hash it returns to a nested array within the current scope's result-hash, under the key 'IDENTIFIER_2'.


Repeatedly call the first subrule. Keep matching as long as the subrule matches, provided successive matches are separated by matches of the second subrule or the pattern.

In other words, match a list of ANY_SUBRULE's separated by ANY_OTHER_SUBRULE's or PATTERN's.

Note that, if a pattern is used to specify the separator, it must be specified in some kind of matched parentheses. These may be capturing [(...)], non-capturing [(?:...)], non-backtracking [(?>...)], or any other construct enclosed by an opening and closing paren.

Special variables within grammar actions


These are both aliases for the built-in read-only $^N variable, which always contains the substring matched by the nearest preceding (...) capture. $^N still works perfectly well, but these are provided to improve the readability of code blocks and error messages respectively.


This variable contains the index at which the next match will be attempted within the string being parsed. It is most commonly used in <error:...> or <log:...> directives:

    <rule: ListElem>
        <log: (?{ "Trying words at index $INDEX" })>
        <MATCH=( \w++ )>
        <log: (?{ "Trying digits at index $INDEX" })>
        <MATCH=( \d++ )>
        <error: (?{ "Missing ListElem near index $INDEX" })>

This variable contains all the saved results of any subrules called from the current rule. In other words, subrule calls like:

    <ListElem>  <Separator= (,)>

stores their respective match results in $MATCH{'ListElem'} and $MATCH{'Separator'}.


This variable is an alias for $MATCH{"="}. This is the %MATCH entry for the special "override value". If this entry is defined, its value overrides the usual "return \%MATCH" semantics of a successful rule.


At the start of any code blocks inside any regex, the variable $_ contains the complete string being matched against. The current matching position within that string is given by: pos($_).


This variable stores the current debugging mode (which may be any of: 'off', 'on', 'run', 'continue', 'match', 'step', or 'try'). It is set automatically by the <debug:...> command, but may also be set manually in a code block (which can be useful for conditional debugging). For example:

    <rule: ListElem>

        # Conditionally debug if 'foobar' encountered...
        (?{ $DEBUG = $MATCH{Identifier} eq 'foobar' ? 'step' : 'off' })


See also: the <log: LOGFILE> and <debug: DEBUG_CMD> directives.


  • The additional regex constructs this module provides are implemented by rewriting regular expressions. This is a (safer) form of source filtering, but still subject to all the same limitations and fallibilities of any other macro-based solution.

  • In particular, rewriting the macros involves the insertion of (a lot of) extra capturing parentheses. This means you can no longer assume that particular capturing parens correspond to particular numeric variables: i.e. to $1, $2, $3 etc. If you want to capture directly use Perl 5.10's named capture construct:

        (?<name> [^\W\d]\w* )

    Better still, capture the data in its correct hierarchical context using the module's "named subpattern" construct:

        <name= ([^\W\d]\w*) >
  • No recursive descent parser--including those created with Regexp::Grammars--can directly handle left-recursive grammars with rules of the form:

        <rule: List>
            <List> , <ListElem>

    If you find yourself attempting to write a left-recursive grammar (which Perl 5.10 may or may not complain about, but will never successfully parse with), then you probably need to use the "separated list" construct instead:

        <rule: List>
            <[ListElem]> ** (,)
  • Grammatical parsing with Regexp::Grammars can fail if your grammar places "non-backtracking" directives (i.e. the (?>...) block or the ?+, *+, or ++ repetition specifiers) around a subrule call. The problem appears to be that preventing the regex from backtracking through the in-regex actions that Regexp::Grammars adds causes the module's internal stack to fall out of sync with the regex match.

    For the time being, you need to make sure that grammar rules don't appear inside a "non-backtracking" directive.


Note that (because the author cannot find a way to throw exceptions from within a regex) none of the following diagnostics actually throws an exception.

Instead, these messages are simply written to the specified parser logfile (or to *STDERR, if no logfile is specified).

However, any fatal match-time message will immediately terminate the parser matching and will still set $@ (as if an exception had been thrown and caught at that point in the code). You then have the option to check $@ immediately after matching with the grammar, and rethrow if necessary:

    if ($input =~ $grammar) {
    else {
        die if $@;
Found call to %s, but no %s was defined in the grammar

You specified a call to a subrule for which there was no definition in the grammar. Typically that's either because you forget to define the rule, or because you misspelled either the definition or the subrule call. For example:


    <rule: fiel>            <---- misspelled rule
        <lines>             <---- used but never defined

Regexp::Grammars converts any such subrule call attempt to an instant catastrophic failure of the entire parse, so if your parser ever actually tries to perform that call, Very Bad Things will happen.

Entire parse terminated prematurely while attempting to call non-existent rule: %s

You ignored the previous error and actually tried to call to a subrule for which there was no definition in the grammar. Very Bad Things are now happening. The parser got very upset, took its ball, and went home. See the preceding diagnostic for remedies.

This diagnostic should throw an exception, but can't. So it sets $@ instead, allowing you to trap the error manually if you wish.

Possible invalid subrule call %s

Your grammar contained something of the form:


which you might have intended to be a subrule call, but which didn't correctly parse as one. If it was supposed to be a Regexp::Grammars subrule call, you need to check the syntax you used. If it wasn't supposed to be a subrule call, you can silence the warning by rewriting it and quoting the leading angle:

Possible invalid directive: %s

Your grammar contained something of the form:


but which wasn't a known directive like <rule:...> or <debug:...>. If it was supposed to be a Regexp::Grammars directive, check the spelling of the directive name. If it wasn't supposed to be a directive, you can silence the warning by rewriting it and quoting the leading angle:

Repeated subrule %s will only capture its final match

You specified a subrule call with a repetition qualifier, such as:




Because each subrule call saves its result in a hash entry of the same name, each repeated match will overwrite the previous ones, so only the last match will ultimately be saved. If you want to save all the matches, you need to tell Regexp::Grammars to save the sequence of results as a nested array within the hash entry, like so:




If you really did intend to throw away every result but the final one, you can silence the warning by placing the subrule call inside any kind of parentheses. For example:



    (?: <ListElem> )+


Regexp::Grammars requires no configuration files or environment variables.


This module only works under Perl 5.10 or later.


This module is likely to be incompatible with any other module that automagically rewrites regexes. For example it may conflict with Regexp::DefaultFlags, Regexp::DeferredExecution, or Regexp::Extended.


No bugs have been reported.

Please report any bugs or feature requests to bug-regexp-grammars@rt.cpan.org, or through the web interface at http://rt.cpan.org.


Damian Conway <DCONWAY@CPAN.org>


Copyright (c) 2009, Damian Conway <DCONWAY@CPAN.org>. All rights reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See perlartistic.