-@e wxWidgets: In a non-Unicode build, these character classifications depend on the
-current locale, and correspond to the values return by the ANSI C 'is'
-functions: isalpha, isupper, etc. In Unicode mode they are based on
-Unicode classifications, and are not affected by the current locale.
-There are two special cases of bracket expressions:
-the bracket expressions @b [[::]] and @b [[::]] are constraints, matching empty
-strings at the beginning and end of a word respectively. A word is defined
-as a sequence of word characters that is neither preceded nor followed
-by word characters. A word character is an @e alnum character or an underscore
-(@b _). These special bracket expressions are deprecated; users of AREs should
-use constraint escapes instead (see #Escapes below).
-
-
-@section overview_resyntax_escapes Escapes
-
-Escapes (AREs only),
-which begin with a <tt>@\</tt> followed by an alphanumeric character, come in several
-varieties: character entry, class shorthands, constraint escapes, and back
-references. A <tt>@\</tt> followed by an alphanumeric character but not constituting
-a valid escape is illegal in AREs. In EREs, there are no escapes: outside
-a bracket expression, a <tt>@\</tt> followed by an alphanumeric character merely stands
-for that character as an ordinary character, and inside a bracket expression,
-<tt>@\</tt> is an ordinary character. (The latter is the one actual incompatibility
-between EREs and AREs.)
-Character-entry escapes (AREs only) exist to make
-it easier to specify non-printing and otherwise inconvenient characters
-in REs:
-
-
-
-@b \a
-
-alert (bell) character, as in C
-
-@b \b
-
-backspace, as in C
-
-@b \B
-
-synonym
-for @b \ to help reduce backslash doubling in some applications where there
-are multiple levels of backslash processing
-
-@b \c@e X
-
-(where X is any character)
-the character whose low-order 5 bits are the same as those of @e X, and whose
-other bits are all zero
-
-@b \e
-
-the character whose collating-sequence name is
-'@b ESC', or failing that, the character with octal value 033
-
-@b \f
-
-formfeed, as in C
-
-@b \n
-
-newline, as in C
-
-@b \r
-
-carriage return, as in C
-
-@b \t
-
-horizontal tab, as in C
-
-@b \u@e wxyz
-
-(where @e wxyz is exactly four hexadecimal digits)
-the Unicode
-character @b U+@e wxyz in the local byte ordering
-
-@b \U@e stuvwxyz
-
-(where @e stuvwxyz is
-exactly eight hexadecimal digits) reserved for a somewhat-hypothetical Unicode
-extension to 32 bits
-
-@b \v
-
-vertical tab, as in C are all available.
-
-@b \x@e hhh
-
-(where
-@e hhh is any sequence of hexadecimal digits) the character whose hexadecimal
-value is @b 0x@e hhh (a single character no matter how many hexadecimal digits
-are used).
-
-@b \0
-
-the character whose value is @b 0
-
-@b \@e xy
-
-(where @e xy is exactly two
-octal digits, and is not a @e back reference (see below)) the character whose
-octal value is @b 0@e xy
-
-@b \@e xyz
-
-(where @e xyz is exactly three octal digits, and is
-not a back reference (see below))
-the character whose octal value is @b 0@e xyz
-
-
-
-Hexadecimal digits are '@b 0'-'@b 9', '@b a'-'@b f', and '@b A'-'@b F'. Octal
-digits are '@b 0'-'@b 7'.
-The character-entry
-escapes are always taken as ordinary characters. For example, @b \135 is @b ] in
-ASCII, but @b \135 does not terminate a bracket expression. Beware, however,
-that some applications (e.g., C compilers) interpret such sequences themselves
-before the regular-expression package gets to see them, which may require
-doubling (quadrupling, etc.) the '@b \'.
-Class-shorthand escapes (AREs only) provide
-shorthands for certain commonly-used character classes:
-
-
-
-@b \d
-
-@b [[:digit:]]
-
-@b \s
-
-@b [[:space:]]
-
-@b \w
-
-@b [[:alnum:]_] (note underscore)
-
-@b \D
-
-@b [^[:digit:]]
-
-@b \S
-
-@b [^[:space:]]
-
-@b \W
-
-@b [^[:alnum:]_] (note underscore)
-
-
-
-Within bracket expressions, '@b \d', '@b \s', and
-'@b \w' lose their outer brackets, and '@b \D',
-'@b \S', and '@b \W' are illegal. (So, for example,
-@b [a-c\d] is equivalent to @b [a-c[:digit:]].
-Also, @b [a-c\D], which is equivalent to
-@b [a-c^[:digit:]], is illegal.)
-A constraint escape (AREs only) is a constraint,
-matching the empty string if specific conditions are met, written as an
-escape:
-
-
-
-@b \A
-
-matches only at the beginning of the string
-(see #Matching, below,
-for how this differs from '@b ^')