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Damian Conway

NAME

Text::Balanced - Extract delimited text sequences from strings.

SYNOPSIS

 use Text::Balanced qw (
                        extract_delimited
                        extract_bracketed
                        extract_quotelike
                        extract_codeblock
                       );

 # Extract the initial substring of $text which is delimited by
 # two (unescaped) instances of the first character in $delim.

        ($extracted, $remainder) = extract_delimited($text,$delim);


 # Extract the initial substring of $text which is bracketed
 # with a delimiter(s) specified by $delim (where the string
 # in $delim contains one or more of '(){}[]<>').

        ($extracted, $remainder) = extract_bracketed($text,$delim);


 # Extract the initial substring of $text which represents a
 # Perl "quote or quote-like operation"

        ($extracted, $remainder) = extract_quotelike($text);


 # Extract the initial substring of $text which represents a block
 # of Perl code, bracketed by any of character(s) specified by $delim
 # (where the string $delim contains one or more of '(){}[]<>').

        ($extracted, $remainder) = extract_codeblock($text,$delim);

DESCRIPTION

The various extract_... subroutines may be used to extract a leading delimited string (possibily after skipping a specified prefix string).

In a list context, they all return a list, the first three elements of which are always: the extracted string (including the specified delimiters) , the remainder of the input string, the skipped prefix. Note that the original input text (the first argument) is not modified in any way.

In a scalar context, the extracted string is returned, having first been removed from the input text.

In a void context, the input text has the extracted string (and any specified prefix) removed from it.

extract_delimited

The extract_delimited function formalizes the common idiom of extracting a single-character-delimited substring from the start of a string. For example, to extract a single-quote delimited string, the following code is typically used:

        ($remainder = $text) =~ s/\A('(\\'|[^'])*')//s;
        $extracted = $1;

but with extract_delimited it can be simplified to:

        ($extracted,$remainder) = extract_delimited $text, "'";

extract_delimited takes up to three scalars (the input text, the delimiters, and a prefix to be skipped) and extracts the initial substring of the text which is appropriately delimited. If the delimiter string has multiple characters, the first one encountered in the text is taken to delimit the substring (the delimiters are simply interpolated into a regex character class, so care is needed with some pattern metacharacters). The third argument specifies a prefix pattern which is to be skipped (but must be present!) before the substring is extracted.

All arguments are optional. If the prefix is not specified, the pattern '\s*' - optional whitespace - is used. If the delimiter set is also not specified, the set /["'`]/ is used. If the text to be processed is not specified either, $_ is used.

In list context, extract_delimited returns a array of three elements, the extracted substring (including the surrounding delimiters), the remainder of the text, and the skipped prefix (if any). If a suitable delimited substring is not found, the first element of the array is the empty string, the second is the complete original text, and the prefix returned in the third element is an empty string.

In a scalar context, just the extracted substring is returned. In a void context, the extracted substring (and any prefix) are simply removed from the beginning of the first argument.

Examples:

        # Remove a single-quoted substring from the very beginning of $text:

                $substring = extract_delimited($text, "'", '');

        # Extract a single- or double- quoted substring from the
        # beginning of $text, optionally after some whitespace
        # (note the list context to protect $text from modification):

                ($substring) = extract_delimited $text, q{"'};


        # Delete the substring delimited by the first '/' in $text:

                $text = join '', (extract_delimited($text,'/','[^/]*')[2,1];

Note that this last example is not the same as deleting the first quote-like pattern. For instance, if $text contained the string:

        "if ('./cmd' =~ m/$UNIXCMD/s) { $cmd = $1; }"
        

then after the deletion it would contain:

        "if ('.$UNIXCMD/s) { $cmd = $1; }"

not:

        "if ('./cmd' =~ ms) { $cmd = $1; }"
        

See "extract_quotelike" for a (partial) solution to this problem.

extract_bracketed

Like "extract_delimited", the extract_bracketed function takes up to three optional scalar arguments: a string to extract from, a delimiter specifier, and a prefix pattern. As before, a missing prefix defaults to optional whitespace and a missing text defaults to $_. However, a missing delimiter specifier defaults to '{}()[]<>' (see below).

extract_bracketed extracts a balanced-bracket-delimited substring (using any one (or more) of the user-specified delimiter brackets: '(..)', '{..}', '[..]', or '<..>').

A "delimiter bracket" is a bracket in list of delimiters passed as extract_bracketed's second argument. Delimiter brackets are specified by giving either the left or right (or both!) versions of the required bracket(s). Note that the order in which two or more delimiter brackets are specified is not significant.

A "balanced-bracket-delimited substring" is a substring bounded by matched brackets, such that any other (left or right) delimiter bracket within the substring is also matched by an opposite (right or left) delimiter bracket at the same level of nesting. Any type of bracket not in the delimiter list is treated as an ordinary character.

In other words, each type of bracket specified as a delimiter must be balanced and correctly nested within the substring, and any other kind of ("non-delimiter") bracket in the substring is ignored.

For example, given the string:

        $text = "{ an '[irregularly :-(] {} parenthesized >:-)' string }";

then a call to extract_bracketed in a list context:

        @result = extract_bracketed( $text, '{}' );

would return:

        ( "{ an '[irregularly :-(] {} parenthesized >:-)' string }" , "" , "" )

since both sets of '{..}' brackets are properly nested and evenly balanced. (In a scalar context just the first element of the array would be returned. In a void context, $text would be replaced by an empty string.)

Likewise the call in:

        @result = extract_bracketed( $text, '{[' );

would return the same result, since all sets of both types of specified delimiter brackets are correctly nested and balanced.

However, the call in:

        @result = extract_bracketed( $text, '{([<' );

would fail, returning:

        ( undef , "{ an '[irregularly :-(] {} parenthesized >:-)' string }"  );

because the embedded pairs of '(..)'s and '[..]'s are "cross-nested" and the embedded '>' is unbalanced. (In a scalar context, this call would return an empty string. In a void context, $text would be unchanged.)

Note that the embedded single-quotes in the string don't help in this case, since they treated as non-delimiter characters and therefore ignored. See "extract_quotelike" and "extract_codeblock", which do understand the nesting behaviour of the various species of Perl quotes.

extract_quotelike

extract_quotelike attempts to recognize, extract, and segment any one of the various Perl quotes and quotelike operators (see perlop(3)) Nested backslashed delimiters, embedded balanced bracket delimiters (for the quotelike operators), and trailing modifiers are all caught. For example, in:

        extract_quotelike 'q # an octothorpe: \# (not the end of the q!) #'
        
        extract_quotelike '  "You said, \"Use sed\"."  '

        extract_quotelike ' s{([A-Z]{1,8}\.[A-Z]{3})} /\L$1\E/; '

        extract_quotelike ' tr/\\\/\\\\/\\\//ds; '

the full Perl quotelike operations are all extracted correctly.

Note too that, when using the /x modifier on a regex, any comment containing the current pattern delimiter will cause the regex to be immediately terminated. In other words:

        'm /
                (?i)            # CASE INSENSITIVE
                [a-z_]          # LEADING ALPHABETIC/UNDERSCORE
                [a-z0-9]*       # FOLLOWED BY ANY NUMBER OF ALPHANUMERICS
           /x'

will be extracted as if it were:

        'm /
                (?i)            # CASE INSENSITIVE
                [a-z_]          # LEADING ALPHABETIC/'

This behaviour is identical to that of the Perl 5.004 interpreter.

extract_quotelike takes two arguments: the text to be processed and a prefix to be matched at the very beginning of the text. If no prefix is specified, optional whitespace is the default. If no text is given, $_ is used.

In a list context, an array of 11 elements is returned. The elements are:

[0]

the extracted quotelike substring (including trailing modifiers),

[1]

the remainder of the input text,

[2]

the prefix substring (if any),

[3]

the name of the quotelike operator (if any),

[4]

the left delimiter of the first block of the operation,

[5]

the text of the first block of the operation (that is, the contents of a quote, the regex of a match or substitution or the target list of a translation),

[6]

the right delimiter of the first block of the operation,

[7]

the left delimiter of the second block of the operation (that is, if it is a s, tr, or y),

[8]

the text of the second block of the operation (that is, the replacement of a substitution or the translation list of a translation),

[9]

the right delimiter of the second block of the operation (if any),

[10]

the trailing modifiers on the operation (if any).

For each of the fields marked "(if any)" the default value is an empty string.

In a scalar context, extract_quotelike returns just the complete substring which matched a quotelike operation (or undef on failure). In a scalar or void context, the input text has the same substring (and any specified prefix) removed.

Examples:

        # Remove the first quotelike literal that appears in text

                $quotelike = extract_quotelike($text,'.*?');

        # Replace one or more leading whitespace-separated quotelike
        # literals in $_ with "<QLL>"

                do { $_ = join '<QLL>', (extract_quotelike)[2,1] } until $@;


        # Isolate the search pattern in a quotelike operation from $text

                ($op,$pat) = (extract_quotelike $text)[3,5];
                if ($op =~ /[ms]/)
                {
                        print "search pattern: $pat\n";
                }
                else
                {
                        print "$op is not a pattern matching operation\n";
                }

extract_codeblock

extract_codeblock attempts to recognize and extract a balanced bracket delimited substring which may contain unbalanced brackets inside Perl quotes or quotelike operations. That is, extract_codeblock is like a combination of "extract_bracketed" and "extract_quotelike".

extract_codeblock takes the same three parameters as extract_bracketed: a text to process, a set of delimiter brackets to look for, and a prefix to match first. No prefix argument implies optional whitespace at the start, no delimiter brackets indicates that only '{' is to be used. No input no text means process $_.

Once the prefix has been disposed of, code blocks are extracted by stepping through the text and trying the following alternatives in sequence:

  1. Try and match a closing delimiter bracket. If the bracket was the same species as the last opening bracket, return the substring to that point. If the bracket was mismatched, return an error.

  2. Try to match a quote or quotelike operator. If found, call extract_quotelike to eat it. If extract_quotelike fails, return the error it returned. Otherwise go back to step 1.

  3. Try to match an opening delimiter bracket. If found, call extract_codeblock recursively to eat the embedded block. If the recursive call fails, return an error. Otherwise, go back to step 1.

  4. Unconditionally match a bareword or any other single character, and then go back to step 1.

Examples:

        # Find a while loop in the text

                if ($text =~ s/.*?while\s*\{/{/)
                {
                        $loop = "while " . extract_codeblock($text);
                }

        # Remove the first round-bracketed list (which may include
        # round- or curly-bracketed code blocks or quotelike operators)

                extract_codeblock $text, "(){}", '[^(]*';

DIAGNOSTICS

In a list context, all the functions return (undef,$original_text) on failure. In a scalar context, failure is indicated by returning undef (in this case the input text is not modified in any way).

In addition, on failure in any context, one of the following explanatory diagnostic messages is returned in the standard $@ variable (on success the $@ variable is guaranteed to be undef):

Did not find a suitable bracket: "%s"

The delimiter provided to extract_bracketed was not one of '()[]<>{}'.

Did not find prefix: /%s/

A non-optional prefix was specified but wasn't found at the start of the text.

Could not extract "%c"-delimited substring

extract_delimited, extract_quotelike or extract_codeblock couldn't find an initial substring (after the prefix) which was delimited by the delimiter(s) specified.

Did not find opening bracket after prefix: "%s"

extract_bracketed or extract_codeblock was expecting a particular kind of bracket at the start of the text, and didn't find it.

No quotelike operator found after prefix: "%s"

extract_quotelike didn't find one of the quotelike operators q, qq, qw, qx, s, tr or y at the start of the substring it was extracting.

Unmatched closing bracket: "%c"

extract_bracketed, extract_quotelike or extract_codeblock encountered a closing bracket where none was expected.

Unmatched opening bracket(s): "%s"

extract_bracketed, extract_quotelike or extract_codeblock ran out of characters in the text before closing one or more levels of nested brackets.

Mismatched closing bracket: expected "%c" but found "%s"

extract_bracketed, extract_quotelike or extract_codeblock found a valid bracket delimiter, but it was the wrong species. This usually indicates a nesting error, but may indicate incorrect quoting or escaping.

No block delimiter found after quotelike "%s"

extract_quotelike or extract_codeblock found one of the quotelike operators q, qq, qw, qx, s, tr or y without a suitable block after it.

Missing second block for quotelike "%s"

extract_codeblock or extract_quotelike found one of the quotelike operators s, tr or y followed by only one block.

Nested codeblock failed to balance from "%s..."

A block within the substring being extracted by extract_codeblock was incorrectly nested or had a unmatched delimiter in it.

AUTHOR

Damian Conway (damian@cs.monash.edu.au)

BUGS AND IRRITATIONS

There are undoubtedly serious bugs lurking somewhere in this code, if only because parts of it give the impression of understanding a great deal more about Perl than they really do.

Bug reports and other feedback are most welcome.

COPYRIGHT

 Copyright (c) 1997-1998, Damian Conway. All Rights Reserved.
 This module is free software. It may be used, redistributed
and/or modified under the terms of the Perl Artistic License
     (see http://www.perl.com/perl/misc/Artistic.html)