syntax_perl.qbk

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  Copyright 2006-2007 John Maddock.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
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[section:perl_syntax Perl Regular Expression Syntax]

[h3 Synopsis]

The Perl regular expression syntax is based on that used by the 
programming language Perl .  Perl regular expressions are the 
default behavior in Boost.Regex or you can pass the flag `perl` to the 
[basic_regex] constructor, for example:

   // e1 is a case sensitive Perl regular expression: 
   // since Perl is the default option there's no need to explicitly specify the syntax used here:
   boost::regex e1(my_expression);
   // e2 a case insensitive Perl regular expression:
   boost::regex e2(my_expression, boost::regex::perl|boost::regex::icase);

[h3 Perl Regular Expression Syntax]

In Perl regular expressions, all characters match themselves except for the 
following special characters:

[pre .\[{()\\\*+?|^$]

[h4 Wildcard]

The single character '.' when used outside of a character set will match 
any single character except:

* The NULL character when the [link boost_regex.ref.match_flag_type flag 
   `match_not_dot_null`] is passed to the matching algorithms.
* The newline character when the [link boost_regex.ref.match_flag_type 
   flag `match_not_dot_newline`] is passed to 
   the matching algorithms.
   
[h4 Anchors]

A '^' character shall match the start of a line.

A '$' character shall match the end of a line.

[h4 Marked sub-expressions]

A section beginning `(` and ending `)` acts as a marked sub-expression.  
Whatever matched the sub-expression is split out in a separate field by 
the matching algorithms.  Marked sub-expressions can also repeated, or 
referred to by a back-reference.

[h4 Non-marking grouping]

A marked sub-expression is useful to lexically group part of a regular 
expression, but has the side-effect of spitting out an extra field in 
the result.  As an alternative you can lexically group part of a 
regular expression, without generating a marked sub-expression by using 
`(?:` and `)` , for example `(?:ab)+` will repeat `ab` without splitting 
out any separate sub-expressions.

[h4 Repeats]

Any atom (a single character, a marked sub-expression, or a character class) 
can be repeated with the `*`, `+`, `?`, and `{}` operators.

The `*` operator will match the preceding atom zero or more times, 
for example the expression `a*b` will match any of the following:

   b
   ab
   aaaaaaaab

The `+` operator will match the preceding atom one or more times, for 
example the expression `a+b` will match any of the following:

   ab
   aaaaaaaab

But will not match:

   b

The `?` operator will match the preceding atom zero or one times, for 
example the expression ca?b will match any of the following:

   cb
   cab

But will not match:

   caab

An atom can also be repeated with a bounded repeat:

`a{n}`  Matches 'a' repeated exactly n times.

`a{n,}`  Matches 'a' repeated n or more times.

`a{n, m}`  Matches 'a' repeated between n and m times inclusive.

For example:

[pre ^a{2,3}$]

Will match either of:

   aa
   aaa

But neither of:

   a
   aaaa

It is an error to use a repeat operator, if the preceding construct can not 
be repeated, for example:

   a(*)

Will raise an error, as there is nothing for the `*` operator to be applied to.

[h4 Non greedy repeats]

The normal repeat operators are "greedy", that is to say they will consume as 
much input as possible.  There are non-greedy versions available that will 
consume as little input as possible while still producing a match.

`*?` Matches the previous atom zero or more times, while consuming as little 
   input as possible.

`+?` Matches the previous atom one or more times, while consuming as 
   little input as possible.

`??` Matches the previous atom zero or one times, while consuming 
   as little input as possible.

`{n,}?` Matches the previous atom n or more times, while consuming as 
   little input as possible.

`{n,m}?` Matches the previous atom between n and m times, while 
   consuming as little input as possible.
   
[h4 Back references]

An escape character followed by a digit /n/, where /n/ is in the range 1-9, 
matches the same string that was matched by sub-expression /n/.  For example 
the expression:

[pre ^(a\*).\*\\1$]

Will match the string:

   aaabbaaa

But not the string:

   aaabba

[h4 Alternation]

The `|` operator will match either of its arguments, so for example: 
`abc|def` will match either "abc" or "def". 

Parenthesis can be used to group alternations, for example: `ab(d|ef)` 
will match either of "abd" or "abef".

Empty alternatives are not allowed (these are almost always a mistake), but 
if you really want an empty alternative use `(?:)` as a placeholder, for example:

`|abc` is not a valid expression, but

`(?:)|abc` is and is equivalent, also the expression:

`(?:abc)??` has exactly the same effect.

[h4 Character sets]

A character set is a bracket-expression starting with `[` and ending with `]`, 
it defines a set of characters, and matches any single character that is a 
member of that set.

A bracket expression may contain any combination of the following:

[h5 Single characters]

For example `[abc]`, will match any of the characters 'a', 'b', or 'c'.

[h5 Character ranges]

For example `[a-c]` will match any single character in the range 'a' to 'c'.  
By default, for Perl regular expressions, a character x is within the 
range y to z, if the code point of the character lies within the codepoints of
the endpoints of the range.  Alternatively, if you set the 
[link boost_regex.ref.syntax_option_type.syntax_option_type_perl `collate` flag] 
when constructing the regular expression, then ranges are locale sensitive.

[h5 Negation]

If the bracket-expression begins with the ^ character, then it matches the 
complement of the characters it contains, for example `[^a-c]` matches 
any character that is not in the range `a-c`.

[h5 Character classes]

An expression of the form `[[:name:]]` matches the named character class 
"name", for example `[[:lower:]]` matches any lower case character.  
See [link boost_regex.syntax.character_classes character class names].

[h5 Collating Elements]

An expression of the form `[[.col.]` matches the collating element /col/.  
A collating element is any single character, or any sequence of characters 
that collates as a single unit.  Collating elements may also be used 
as the end point of a range, for example: `[[.ae.]-c]` matches the 
character sequence "ae", plus any single character in the range "ae"-c, 
assuming that "ae" is treated as a single collating element in the current locale.

As an extension, a collating element may also be specified via it's 
[link boost_regex.syntax.collating_names symbolic name], for example:

   [[.NUL.]]

matches a `\0` character.

[h5 Equivalence classes]

An expression of the form `[[=col=]]`, matches any character or collating element 
whose primary sort key is the same as that for collating element /col/, as with 
collating elements the name /col/ may be a 
[link boost_regex.syntax.collating_names symbolic name].  A primary sort key is 
one that ignores case, accentation, or locale-specific tailorings; so for 
example `[[=a=]]` matches any of the characters: 
a, '''À''', '''Á''', '''Â''', 
'''Ã''', '''Ä''', '''Å''', A, '''à''', '''á''', 
'''â''', '''ã''', '''ä''' and '''å'''.  
Unfortunately implementation of this is reliant on the platform's collation 
and localisation support; this feature can not be relied upon to work portably 
across all platforms, or even all locales on one platform.

[h5 Escaped Characters]

All the escape sequences that match a single character, or a single character 
class are permitted within a character class definition.  For example
`[\[\]]` would match either of `[` or `]` while `[\W\d]` would match any character
that is either a "digit", /or/ is /not/ a "word" character.

[h5 Combinations]

All of the above can be combined in one character set declaration, for example: 
`[[:digit:]a-c[.NUL.]]`.

[h4 Escapes]

Any special character preceded by an escape shall match itself.

The following escape sequences are all synonyms for single characters: