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DRAFT: Synopsis 32: Setting Library - Str


    Rod Adams <rod@rodadams.net>
    Larry Wall <larry@wall.org>
    Aaron Sherman <ajs@ajs.com>
    Mark Stosberg <mark@summersault.com>
    Carl Mäsak <cmasak@gmail.com>
    Moritz Lenz <moritz@faui2k3.org>
    Tim Nelson <wayland@wayland.id.au>


    Created: 19 Mar 2009 extracted from S29-functions.pod

    Last Modified: 17 Apr 2009
    Version: 3

The document is a draft.

If you read the HTML version, it is generated from the Pod in the specs repository under https://github.com/perl6/specs/blob/master/S32-setting-library/Str.pod so edit it there in the git repository if you would like to make changes.


General notes about strings:

A Str can exist at several Unicode levels at once. Which level you interact with typically depends on what your current lexical context has declared the "working Unicode level to be". Default is Grapheme. [Default can't be CharLingua because we don't go into "language" mode unless there's a specific language declaration saying either exactly what language we're going into or, in the absence of that, how to find the exact language somewhere in the environment.]

Attempting to use a string at a level higher it can support is handled without warning. The current highest supported level of the string is simply mapped Char for Char to the new higher level. However, attempting to stuff something of a higher level a lower-level string is an error (for example, attempting to store Kanji in a Byte string). An explicit conversion function must be used to tell it how you want it encoded.

Attempting to use a string at a level lower than what it supports is not allowed.

If a function takes a Str and returns a Str, the returned Str will support the same levels as the input, unless specified otherwise.

The following are all provided by the Str role:

 our Char multi method p5chop ( Str  $string is rw: ) is export(:P5)
 my Char multi p5chop ( Str *@strings is rw ) is export(:P5)

Trims the last character from $string, and returns it. Called with a list, it chops each item in turn, and returns the last character chopped.

 our Str multi method chop ( Str  $string: ) is export

Returns string with one Char removed from the end.

 our Int multi method p5chomp ( Str  $string is rw: ) is export(:P5)
 my Int multi p5chomp ( Str *@strings is rw ) is export(:P5)

Related to p5chop, only removes trailing chars that match /\n/. In either case, it returns the number of chars removed.

 our Str multi method chomp ( Str $string: ) is export

Returns string with one newline removed from the end. An arbitrary terminator can be removed if the input filehandle has marked the string for where the "newline" begins. (Presumably this is stored as a property of the string.) Otherwise a standard newline is removed.

Note: Most users should just let their I/O handles autochomp instead. (Autochomping is the default.)

 our Str multi method lc ( Str $string: ) is export

Returns the input string after converting each character to its lowercase form, if uppercase.

 our Str multi method lcfirst ( Str $string: ) is export

Like lc, but only affects the first character.

 our Str multi method uc ( Str $string: ) is export

Returns the input string after converting each character to its uppercase form, if lowercase. This is not a Unicode "titlecase" operation, but a full "uppercase".

 our Str multi method ucfirst ( Str $string: ) is export

Performs a Unicode "titlecase" operation on the first character of the string.

 our Str multi method normalize ( Str $string: Bool :$canonical = Bool::True, Bool :$recompose = Bool::False ) is export

Performs a Unicode "normalization" operation on the string. This involves decomposing the string into its most basic combining elements, and potentially re-composing it. Full detail on the process of decomposing and re-composing strings in a normalized form is covered in the Unicode specification Sections 3.7, Decomposition and 3.11, Canonical Ordering Behavior of the Unicode Standard, 4.0. Additional named parameters are reserved for future Unicode expansion.

For everyday use there are aliases that map to the Unicode Standard Annex #15: Unicode Normalization Forms document's names for the various modes of normalization:

 our Str multi method nfd ( Str $string: ) is export {
   $string.normalize(:canonical, :!recompose);
 our Str multi method nfc ( Str $string: ) is export {
   $string.normalize(:canonical, :recompose);
 our Str multi method nfkd ( Str $string: ) is export {
   $string.normalize(:!canonical, :!recompose);
 our Str multi method nfkc ( Str $string: ) is export {
   $string.normalize(:!canonical, :recompose);

Decomposing a string can be used to compare Unicode strings in a binary form, providing that they use the same encoding. Without decomposing first, two Unicode strings may contain the same text, but not the same byte-for-byte data, even in the same encoding. The decomposition of a string is performed according to tables in the Unicode standard, and should be compatible with decompositions performed by any system.

The :canonical flag controls the use of "compatibility decompositions". For example, in canonical mode, "fi" is left unaffected because it is not a composition. However, in compatibility mode, it will be replaced with "fi". Decomposed sequences will be ordered in a canonical way in either mode.

The :recompose flag controls the re-composition of decomposed forms. That is, a combining sequence will be re-composed into the canonical composite where possible.

These de-compositions and re-compositions are performed recursively, until there is no further work to be done.

Note that this function is really only applicable when dealing with codepoint strings. Grapheme strings are normally processed at a higher abstraction level that is independent of normalization, and are lazily normalized into the desired normalization when transferred to lexical scopes or handles that care.

 our Str multi method samecase ( Str $string: Str $pattern ) is export

Has the effect of making the case of the string match the case pattern in $pattern. (Used by s:ii/// internally, see S05.)

 our Str multi method samemark ( Str $string: Str $pattern ) is export

Has the effect of making the case of the string match the marking pattern in $pattern. (Used by s:mm/// internally, see S05.)

 our Str multi method capitalize ( Str $string: ) is export

Has the effect of first doing an lc on the entire string, then performing a s:g/(\w+)/{ucfirst $0}/ on it.


This word is banned in Perl 6. You must specify units.

 our Int multi method chars ( Str $string: ) is export

Returns the number of characters in the string in the current (lexically scoped) idea of what a normal character is, usually graphemes.

 our Int multi method codes ( Str $string: ) is export

Returns the number of graphemes in the string in a language-independent way.

 our Int multi method codes ( Str $string: $nf = $?NF) is export

Returns the number of codepoints in the string if it were canonicalized the specified way. Do not confuse codepoints with UTF-16 encoding. Characters above U+FFFF count as a single codepoint.

 our Int multi method bytes ( Str $string: $enc = $?ENC, :$nf = $?NF) is export

Returns the number of bytes in the string if it were encoded in the specified way. Note the inequality:

    .bytes("UTF-16","C") >= .codes("C") * 2

This is caused by the possibility of surrogate pairs, which are counted as one codepoint. However, this problem does not arise for UTF-32:

    .bytes("UTF-32","C") == .codes("C") * 4
    our Buf multi method encode($encoding = $?ENC, $nf = $?NF)

Returns a Buf which represents the original string in the given encoding and normal form. The actual return type is as specific as possible, so $str.encode('UTF-8') returns an utf8 object, $str.encode('ISO-8859-1') a buf8.

 our StrPos multi method index( Str $string: Str $substring, StrPos $pos = StrPos(0) ) is export
 our StrPos multi method index( Str $string: Str $substring, Int $pos ) is export

index searches for the first occurrence of $substring in $string, starting at $pos. If $pos is an Int, it is taken to be in the units of the calling scope, which defaults to "graphemes".

The value returned is always a StrPos object. If the substring is found, then the StrPos represents the position of the first character of the substring. If the substring is not found, a bare StrPos containing no position is returned. This prototype StrPos evaluates to false because it's really a kind of undefined value. Do not evaluate as a number, because instead of returning -1 it will return 0 and issue a warning.

 our buf8 multi pack( *@items where { all(@items) ~~ Pair } )
 our buf8 multi pack( Str $template, *@items )

pack takes a list of pairs and formats the values according to the specification of the keys. Alternately, it takes a string $template and formats the rest of its arguments according to the specifications in the template string. The result is a sequence of bytes.

Templates are strings of the form:

  grammar Str::PackTemplate {
    regex TOP       { ^ <template> $ }
    regex template  { [ <group> | <specifier> <count>? ]* }
    token group     { \( <template> \) }
    token specifier { <[aAZbBhHcCsSiIlLnNvVqQjJfdFDpPuUwxX\@]> \!? }
    token count     { \*
                    | \[ [ \d+ | <specifier> ] \]
                    | \d+ }

In the pairwise mode, each key must contain a single <group> or <specifier>, and the values must be either scalar arguments or arrays.

[ Note: Need more documentation and need to figure out what Perl 5 things no longer make sense. Does Perl 6 need any extra formatting features? -ajs ]

[I think pack formats should be human readable but compiled to an internal form for efficiency. I also think that compact classes should be able to express their serialization in pack form if asked for it with .packformat or some such. -law]

 our Str multi method quotemeta ( Str $string: ) is export

Returns the input string with all non-"word" characters back-slashed. That is, all characters not matching "/<[A..Za..z_0..9]>/" will be preceded by a backslash in the returned string, regardless of any locale settings.

[Note from Pm: Should that be "/\w/" instead? Or, if the intent is to duplicate p5 functionality, perhaps it should be "p5quotemeta"? Do we even want this method at all?]

 our StrPos multi method rindex( Str $string: Str $substring, StrPos $pos? ) is export
 our StrPos multi method rindex( Str $string: Str $substring, Int $pos ) is export

Returns the position of the last $substring in $string. If $pos is specified, then the search starts at that location in $string, and works backwards. See index for more detail.

 our List multi split ( Str $delimiter, Str $input, Int $limit = Inf )
 our List multi split ( Regex $delimiter, Str $input, Int $limit = Inf )
 our List multi method split ( Str $input: Str $delimiter, Int $limit = Inf )
 our List multi method split ( Str $input: Regex $delimiter, Int $limit = Inf, Bool :$all = False)

Splits a string up into pieces based on delimiters found in the string.

String delimiters must not be treated as rules but as constants. The default is no longer ' ' since that would be interpreted as a constant. P5's split(' ') will translate to comb. Null trailing fields are no longer trimmed by default.

The split function no longer has a default delimiter nor a default invocant. In general you should use words to split on whitespace now, or comb to break into individual characters. See below.

If the :all adverb is supplied to the Regex form, then the delimiters are returned as Match objects in alternation with the split values. Unlike with Perl 5, if the delimiter contains multiple captures they are returned as submatches of single Match object. (And since Match does Capture, whether these Match objects eventually flatten or not depends on whether the expression is bound into a list or slice context.)

You may also split lists and filehandles. $*ARGS.split(/\n[\h*\n]+/) splits on paragraphs, for instance. Lists and filehandles are automatically fed through cat in order to pretend to be string. The resulting Cat is lazy. Accessing a filehandle as both a filehandle and as a Cat is undefined.

 our List multi comb ( Regex $matcher, Str $input, Int $limit = Inf )
 our List multi method comb ( Str $input: Regex $matcher = /./, Int $limit = Inf )

The comb function looks through a string for the interesting bits, ignoring the parts that don't match. In other words, it's a version of split where you specify what you want, not what you don't want.

That means the same restrictions apply to the matcher rule as do to split's delimiter rule.

By default it pulls out all individual characters. Saying

    $string.comb(/pat/, $n)

is equivalent to

    map {.Str}, $string.match(rx:global:x(0..$n):c/pat/)

You may also comb lists and filehandles. +$*IN.comb counts the words on standard input, for instance. comb(/./, $thing) returns a list of single Char strings from anything that can give you a Str. Lists and filehandles are automatically fed through cat in order to pretend to be string. This Cat is also lazy.

If the :match adverb is applied, a list of Match objects (one per match) is returned instead of strings. This can be used to access capturing subrules in the matcher. The unmatched portions are never returned -- if you want that, use split :all. If the function is combing a lazy structure, the return values may also be lazy. (Strings are not lazy, however.)

 our List multi method lines ( Str $input: Int $limit = Inf ) is export

Returns a list of lines, i.e. the same as a call to $input.comb( / ^^ \N* /, $limit ) would.

 our List multi method words ( Str $input: Int $limit = Inf ) is export

Returns a list of non-whitespace bits, i.e. the same as a call to $input.comb( / \S+ /, $limit ) would.


The flip function reverses a string character by character.

     our Str multi method flip ( $str: ) is export {

This function will misplace accents if used at a Unicode level less than graphemes.

 our Str multi method sprintf ( Str $format: *@args ) is export

This function is mostly identical to the C library sprintf function.

The $format is scanned for % characters. Any % introduces a format token. Format tokens have the following grammar:

 grammar Str::SprintfFormat {
  regex format_token { '%': <index>? <precision>? <modifier>? <directive> }
  token index { \d+ '$' }
  token precision { <flags>? <vector>? <precision_count> }
  token flags { <[ \x20 + 0 \# \- ]>+ }
  token precision_count { [ <[1..9]>\d* | '*' ]? [ '.' [ \d* | '*' ] ]? }
  token vector { '*'? v }
  token modifier { < ll l h m V q L > }
  token directive { < % c s d u o x e f g X E G b p n i D U O F > }

Directives guide the use (if any) of the arguments. When a directive (other than %) is used, it indicates how the next argument passed is to be formatted into the string.

The directives are:

 %   a literal percent sign
 c   a character with the given codepoint
 s   a string
 d   a signed integer, in decimal
 u   an unsigned integer, in decimal
 o   an unsigned integer, in octal
 x   an unsigned integer, in hexadecimal
 e   a floating-point number, in scientific notation
 f   a floating-point number, in fixed decimal notation
 g   a floating-point number, in %e or %f notation
 X   like x, but using upper-case letters
 E   like e, but using an upper-case "E"
 G   like g, but with an upper-case "E" (if applicable)
 b   an unsigned integer, in binary
 C   special: invokes the arg as code, see below


 i   a synonym for %d
 D   a synonym for %ld
 U   a synonym for %lu
 O   a synonym for %lo
 F   a synonym for %f

Perl 5 (non-)compatibility:

 n   produces a runtime exception (see below)
 p   produces a runtime exception

The special format directive, %C invokes the target argument as code, passing it the result string that has been generated thus far and the argument array.

Here's an example of its use:

 sprintf "%d%C is %d digits long",
    sub ($s, @args is rw) { @args[2] = $s.elems },

The special directive, %n does not work in Perl 6 because of the difference in parameter passing conventions, but the example above simulates its effect using %C.

Modifiers change the meaning of format directives. The most important being support for complex numbers (a basic type in Perl). Here are all of the modifiers and what they modify:

 h interpret integer as native "short" (typically int16)
 l interpret integer as native "long" (typically int32 or int64)
 ll interpret integer as native "long long" (typically int64)
 L interpret integer as native "long long" (typically uint64)
 q interpret integer as native "quads" (typically int64 or larger)
 m interpret value as a complex number

The m modifier works with d,u,o,x,F,E,G,X,E and G format directives, and the directive applies to both the real and imaginary parts of the complex number.


 sprintf "%ld a big number, %lld a bigger number, %mf complexity\n",
    4294967295, 4294967296, 1+2i);
  our Str multi method fmt( Scalar $scalar: Str $format = '%s' )
  our Str multi method fmt( List $list: Str $format = '%s',
                                        Str $separator = ' ' )
  our Str multi method fmt( Hash $hash: Str $format = "%s\t%s",
                                        Str $separator = "\n" )
  our Str multi method fmt( Pair $pair: Str $format = "%s\t%s" )

A set of wrappers around sprintf. A call to the scalar version $o.fmt($format) returns the result of sprintf($format, $o). A call to the list version @a.fmt($format, $sep) returns the result of join $sep, map { sprintf($format, $_) }, @a. A call to the hash version %h.fmt($format, $sep) returns the result of join $sep, map { sprintf($format, $_.key, $_.value) }, %h.pairs. A call to the pair version $p.fmt($format) returns the result of sprintf($format, $p.key, $p.value).

 our Str multi method substr (Str $string: StrPos $start, StrLen $length?) is rw is export
 our Str multi method substr (Str $string: StrPos $start, StrPos $end) is rw is export
 our Str multi method substr (Str $string: StrPos $start, Int $length) is rw is export
 our Str multi method substr (Str $string: Int $start, StrLen $length?) is rw is export
 our Str multi method substr (Str $string: Int $start, StrPos $end) is rw is export
 our Str multi method substr (Str $string: Int $start, Int $length) is rw is export

substr returns part of an existing string. You control what part by passing a starting position and optionally either an end position or length. If you pass a number as either the position or length, then it will be used as the start or length with the assumption that you mean "chars" in the current Unicode abstraction level, which defaults to graphemes. A number in the 3rd argument is interpreted as a length rather than a position (just as in Perl 5).

Here is an example of its use:

 $initials = substr($first_name,0,1) ~ substr($last_name,0,1);

Optionally, you can use substr on the left hand side of an assignment like so:

 $string ~~ /(barney)/;
 substr($string, $0.from, $0.to) = "fred";

If the replacement string is longer or shorter than the matched sub-string, then the original string will be dynamically resized.

  multi method trim() is export;

Returns a copy of the string, with leading and trailing whitespace removed.

 method Match match(Str $self: Regex $search);

See "Substitution" in S05

 method Str subst(Str $self: Regex $search, Str $replacement);
 method trans(Str $self: Str $key, Str $val);

 our multi trans(List of Pair %data);
     our Str multi method indent ($str: $steps) is export;

Returns a re-indented string wherein $steps number of spaces have been added to each line (i.e. to the beginning of the string and after each logical newline).

If $steps is negative, removes that many spaces instead. Should any line contain too few leading spaces, only those are removed and a warning is issued.

If $steps is *, removes exactly as many spaces as are needed to make at least one line have zero indentation.

When removing indentation, the method will assume hard tabs to be ($?TABSTOP // 8) spaces, and will treat other horizontal whitespace characters as synonymous to spaces. Characters not participating in the re-indenting will be left untouched, and those added in an indent call will be either (1) consistent with subsequent leading whitespace already on the line, if these are all the same, or (2) spaces. During an unindent, the trailing tab character in a chain of leading tab characters may explode into a number of space characters.


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