Nim/doc/regexprs.txt

Licence of the PCRE library
===========================

PCRE is a library of functions to support regular expressions whose
syntax and semantics are as close as possible to those of the Perl 5
language.

| Written by Philip Hazel
| Copyright (c) 1997-2005 University of Cambridge

----------------------------------------------------------------------

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Regular expression syntax and semantics
=======================================

As the regular expressions supported by this module are enormous,
the reader is referred to http://perldoc.perl.org/perlre.html for the
full documentation of Perl's regular expressions.

Because the backslash ``\`` is a meta character both in the Nim
programming language and in regular expressions, it is strongly
recommended that one uses the *raw* strings of Nim, so that
backslashes are interpreted by the regular expression engine:

```nim
  r"\S"  # matches any character that is not whitespace
```

A regular expression is a pattern that is matched against a subject string
from left to right. Most characters stand for themselves in a pattern, and
match the corresponding characters in the subject. As a trivial example,
the pattern:

    The quick brown fox

matches a portion of a subject string that is identical to itself.
The power of regular expressions comes from the ability to include
alternatives and repetitions in the pattern. These are encoded in
the pattern by the use of metacharacters, which do not stand for
themselves but instead are interpreted in some special way.

There are two different sets of metacharacters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized in square brackets. Outside square brackets, the metacharacters
are as follows:

==============     ============================================================
meta character     meaning
==============     ============================================================
``\``              general escape character with several uses
``^``              assert start of string (or line, in multiline mode)
``$``              assert end of string (or line, in multiline mode)
``.``              match any character except newline (by default)
``[``              start character class definition
``|``              start of alternative branch
``(``              start subpattern
``)``              end subpattern
``{``              start min/max quantifier
``?``              extends the meaning of ``(``
                   | also 0 or 1 quantifier (equal to ``{0,1}``)
                   | also quantifier minimizer
``*``              0 or more quantifier (equal to ``{0,}``)
``+``              1 or more quantifier (equal to ``{1,}``)
                   | also "possessive quantifier"
==============     ============================================================


Part of a pattern that is in square brackets is called a "character class".
In a character class the only metacharacters are:

==============     ============================================================
meta character     meaning
==============     ============================================================
``\``              general escape character
``^``              negate the class, but only if the first character
``-``              indicates character range
``[``              POSIX character class (only if followed by POSIX syntax)
``]``              terminates the character class
==============     ============================================================


The following sections describe the use of each of the metacharacters.


Backslash
---------
The `backslash`:idx: character has several uses. Firstly, if it is followed
by a non-alphanumeric character, it takes away any special meaning that
character may have. This use of backslash as an escape character applies
both inside and outside character classes.

For example, if you want to match a ``*`` character, you write ``\*`` in
the pattern. This escaping action applies whether or not the following
character would otherwise be interpreted as a metacharacter, so it is always
safe to precede a non-alphanumeric with backslash to specify that it stands
for itself. In particular, if you want to match a backslash, you write ``\\``.


Non-printing characters
-----------------------
A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually easier to
use one of the following escape sequences than the binary character it
represents:

==============     ============================================================
character          meaning
==============     ============================================================
``\a``             alarm, that is, the BEL character (hex 07)
``\e``             escape (hex 1B)
``\f``             formfeed (hex 0C)
``\n``             newline (hex 0A)
``\r``             carriage return (hex 0D)
``\t``             tab (hex 09)
``\ddd``           character with octal code ddd, or backreference
``\xhh``           character with hex code hh
==============     ============================================================

After ``\x``, from zero to two hexadecimal digits are read (letters can be in
upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
appear between ``\x{`` and ``}``, but the value of the character code must be
less than 2^31 (that is, the maximum hexadecimal value is 7FFFFFFF). If
characters other than hexadecimal digits appear between ``\x{`` and ``}``, or
if there is no terminating ``}``, this form of escape is not recognized.
Instead, the initial ``\x`` will be interpreted as a basic hexadecimal escape,
with no following digits, giving a character whose value is zero.

After ``\0`` up to two further octal digits are read. In both cases, if there
are fewer than two digits, just those that are present are used. Thus the
sequence ``\0\x\07`` specifies two binary zeros followed by a BEL character
(code value 7). Make sure you supply two digits after the initial zero if
the pattern character that follows is itself an octal digit.

The handling of a backslash followed by a digit other than 0 is complicated.
Outside a character class, PCRE reads it and any following digits as a
decimal number. If the number is less than 10, or if there have been at least
that many previous capturing left parentheses in the expression, the entire
sequence is taken as a back reference. A description of how this works is
given later, following the discussion of parenthesized subpatterns.

Inside a character class, or if the decimal number is greater than 9 and
there have not been that many capturing subpatterns, PCRE re-reads up to
three octal digits following the backslash, and generates a single byte
from the least significant 8 bits of the value. Any subsequent digits stand
for themselves. For example:

==============     ============================================================
example            meaning
==============     ============================================================
``\040``           is another way of writing a space
``\40``            is the same, provided there are fewer than 40 previous
                   capturing subpatterns
``\7``             is always a back reference
``\11``            might be a back reference, or another way of writing a tab
``\011``           is always a tab
``\0113``          is a tab followed by the character "3"
``\113``           might be a back reference, otherwise the character with
                   octal code 113
``\377``           might be a back reference, otherwise the byte consisting
                   entirely of 1 bits
``\81``            is either a back reference, or a binary zero followed by
                   the two characters "8" and "1"
==============     ============================================================

Note that octal values of 100 or greater must not be introduced by a leading
zero, because no more than three octal digits are ever read.

All the sequences that define a single byte value or a single UTF-8 character
(in UTF-8 mode) can be used both inside and outside character classes. In
addition, inside a character class, the sequence ``\b`` is interpreted as the
backspace character (hex 08), and the sequence ``\X`` is interpreted as the
character "X". Outside a character class, these sequences have different
meanings (see below).

Generic character types
-----------------------
The third use of backslash is for specifying `generic character types`:idx:.
The following are always recognized:

==============     ============================================================
character type     meaning
==============     ============================================================
``\d``             any decimal digit
``\D``             any character that is not a decimal digit
``\s``             any whitespace character
``\S``             any character that is not a whitespace character
``\w``             any "word" character
``\W``             any "non-word" character
==============     ============================================================

Each pair of escape sequences partitions the complete set of characters into
two disjoint sets. Any given character matches one, and only one, of each pair.

These character type sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the
current matching point is at the end of the subject string, all of them fail,
since there is no character to match.

For compatibility with Perl, ``\s`` does not match the VT character (code 11).
This makes it different from the POSIX "space" class. The ``\s`` characters
are HT (9), LF (10), FF (12), CR (13), and space (32).

A "word" character is an underscore or any character less than 256 that is
a letter or digit. The definition of letters and digits is controlled by
PCRE's low-valued character tables, and may vary if locale-specific matching
is taking place (see "Locale support" in the pcreapi page). For example,
in the "fr_FR" (French) locale, some character codes greater than 128 are
used for accented letters, and these are matched by ``\w``.

In UTF-8 mode, characters with values greater than 128 never match ``\d``,
``\s``, or ``\w``, and always match ``\D``, ``\S``, and ``\W``. This is true
even when Unicode character property support is available.

Simple assertions
-----------------
The fourth use of backslash is for certain `simple assertions`:idx:. An
assertion specifies a condition that has to be met at a particular point in
a match, without consuming any characters from the subject string. The use of
subpatterns for more complicated assertions is described below. The
backslashed assertions are:

==============     ============================================================
assertion          meaning
==============     ============================================================
``\b``             matches at a word boundary
``\B``             matches when not at a word boundary
``\A``             matches at start of subject
``\Z``             matches at end of subject or before newline at end
``\z``             matches at end of subject
``\G``             matches at first matching position in subject
==============     ============================================================

These assertions may not appear in character classes (but note that ``\b``
has a different meaning, namely the backspace character, inside a character
class).

A word boundary is a position in the subject string where the current
character and the previous character do not both match ``\w`` or ``\W`` (i.e.
one matches ``\w`` and the other matches ``\W``), or the start or end of the
string if the first or last character matches ``\w``, respectively.

The ``\A``, ``\Z``, and ``\z`` assertions differ from the traditional
circumflex and dollar in that they only ever match at the very start and
end of the subject string, whatever options are set.
The difference between ``\Z`` and ``\z`` is that ``\Z`` matches before
a newline that is the last character of the string as well as at the end
of the string, whereas ``\z`` matches only at the end.