Regular expressions are expressions consisting of elements resembling those of
numeric expressions. Regular expressions consist of basic elements and
operators, having various priorities and associations. Like numeric
expressions, parentheses can be used to group elements together to form a
unit on which operators operate. For an extensive discussion the reader is
referred to, e.g., section 15.10 of the url(ecma-international.org)
    (http://ecma-international.org/ecma-262/5.1/#sec-15.10) page, which
describes the characteristics of the regular expressions used by default by
bf(C++)'s tt(regex) classes.

bf(C++)'s default definition of regular expressions distinguishes the
following em(atoms):
    itemization(
    itt(x): the character `x';

    itt(.): any character except for the newline character;

    itt([xyz]): a character class; in this case, either an `x', a `y', or a `z'
        matches the regular expression.  See also the paragraph about
        character classes below;

    itt([abj-oZ]): a character class containing a range of characters; this
        regular expression matches an `a', a `b', any letter from `j' through
        `o', or a `Z'.  See also the paragraph about character classes below;

    itt([^A-Z]): a negated character class: this regular expression matches
        any character but those in the class beyond tt(^).  In this case, any
        character em(except for) an uppercase letter.  See also the paragraph
        about character classes below;

    itt([:predef:]): a em(predefined) set of characters. See below for an
        overview. When used, it is interpreted as an element in a character
        class. It is therefore always embedded in a set of square brackets
        defining the character class (e.g., tt([[:alnum:]]));

    itt(\X): if X is `a', `b', `f', `n', `r', `t', or `v', then the ANSI-C
        interpretation of `\x'. Otherwise, a literal `X' (used to escape
        operators such as tt(*));

    itt((r)): the regular expression tt(r). It is used to override precedence
        (see below), but also to define tt(r) as a
       emi(marked sub-expression) whose matching characters may directly be
        retrieved from, e.g., a tt(std::smatch) object (cf. section
        ref(SMATCH));

    itt((?:r)): the regular expression tt(r). It is used to override
        precedence (see below), but it is em(not) regarded as a em(marked
        sub-expression);
    )

In addition to these basic atoms, the following special atoms are available
(which can also be used in character classes):
    itemization(
    itt(\s): a whitespace character;

    itt(\S): any character but a whitespace character;

    itt(\d): a decimal digit character;

    itt(\D): any character but a decimal digit character;

    itt(\w): an alphanumeric character or an underscore (tt(_)) character;

    itt(\W): any character but an alphanumeric character or an underscore
        (tt(_)) character.
    )

Atoms may be concatenated. If tt(r) and tt(s) are atoms then the regular
expression tt(rs) matches a target text if the target text matches tt(r)
em(and) tt(s), in that order (without any intermediate characters 
inside the target text). E.g., the regular expression tt([ab][cd]) matches the
target text tt(ac), but not the target text tt(a:c). 

Atoms may be combined using operators. Operators bind to the preceding
atom. If an operator should operate on multiple atoms the atoms must be
surrounded by parentheses (see the last element in the previous itemization). 
To use an operator character as an atom it can be escaped. Eg., tt(*)
represents an operator, tt(\*) the atom character star. Note that 
character classes do not recognize escape sequences: tt([\*]) represents a
character class consisting of two characters: a backslash and a star.

The following operators are supported (tt(r) and tt(s) represent regular
expression atoms):
    itemization(
    itt(r*): zero or more tt(r)s;

    itt(r+): one or more tt(r)s;

    itt(r?): zero or one tt(r)s (that is, an optional r);

    itt(r{m, n}): where tt(1 <= m <= n): matches `r' at least m, but at most n
        times;

    itt(r{m,}): where tt(1 <= m): matches `r' at least m times;

    itt(r{m}): where tt(1 <= m): matches `r' exactly m times;

    itt(r|s): matches either an `r' or an `s'. This operator has a lower
        priority than any of the multiplication operators;

    itt(^r) : tt(^) is a pseudo operator. This expression matches `r', if
        appearing at the beginning of the target text. If the tt(^)-character
        is not the first character of a regular expression it is interpreted
        as a literal tt(^)-character;

    itt(r$): tt($) is a pseudo operator. This expression matches `r', if
        appearing at the end of the target text. If the tt($)-character is not
        the last character of a regular expression it is interpreted as a
        literal tt($)-character;
    )

When a regular expression contains marked sub-expressions and multipliers, and
the marked sub-expressions are multiply matched, then the target's final
sub-string matching the marked sub-expression is reported as the text matching
the marked sub-expression. E.g, when using tt(regex_search) (cf. section
ref(REGSRCH)), marked sub-expression (tt(((a|b)+\s?))), and target text tt(a a
b), then tt(a a b) is the fully matched text, while tt(b) is reported as the
sub-string matching the first and second marked sub-expressions.







