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3.7 'gawk'-Specific Regexp Operators
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GNU software that deals with regular expressions provides a number of
additional regexp operators.  These operators are described in this
minor node and are specific to 'gawk'; they are not available in other
'awk' implementations.  Most of the additional operators deal with word
matching.  For our purposes, a "word" is a sequence of one or more
letters, digits, or underscores ('_'):

'\s'
     Matches any space character as defined by the current locale.
     Think of it as shorthand for '[[:space:]]'.

'\S'
     Matches any character that is not a space, as defined by the
     current locale.  Think of it as shorthand for '[^[:space:]]'.

'\w'
     Matches any word-constituent character--that is, it matches any
     letter, digit, or underscore.  Think of it as shorthand for
     '[[:alnum:]_]'.

'\W'
     Matches any character that is not word-constituent.  Think of it as
     shorthand for '[^[:alnum:]_]'.

'\<'
     Matches the empty string at the beginning of a word.  For example,
     '/\'
     Matches the empty string at the end of a word.  For example,
     '/stow\>/' matches 'stow' but not 'stowaway'.

'\y'
     Matches the empty string at either the beginning or the end of a
     word (i.e., the word boundar*y*).  For example, '\yballs?\y'
     matches either 'ball' or 'balls', as a separate word.

'\B'
     Matches the empty string that occurs between two word-constituent
     characters.  For example, '/\Brat\B/' matches 'crate', but it does
     not match 'dirty rat'.  '\B' is essentially the opposite of '\y'.
     Another way to think of this is that '\B' matches the empty string
     provided it's not at the edge of a word.

   There are two other operators that work on buffers.  In Emacs, a
"buffer" is, naturally, an Emacs buffer.  Other GNU programs, including
'gawk', consider the entire string to match as the buffer.  The
operators are:

'\`'
     Matches the empty string at the beginning of a buffer (string)

'\''
     Matches the empty string at the end of a buffer (string)

   Because '^' and '$' always work in terms of the beginning and end of
strings, these operators don't add any new capabilities for 'awk'.  They
are provided for compatibility with other GNU software.

   In other GNU software, the word-boundary operator is '\b'.  However,
that conflicts with the 'awk' language's definition of '\b' as
backspace, so 'gawk' uses a different letter.  An alternative method
would have been to require two backslashes in the GNU operators, but
this was deemed too confusing.  The current method of using '\y' for the
GNU '\b' appears to be the lesser of two evils.

   The various command-line options (*note Options::) control how 'gawk'
interprets characters in regexps:

No options
     In the default case, 'gawk' provides all the facilities of POSIX
     regexps and the GNU regexp operators described in *note Regexp
     Operators::.

'--posix'
     Match only POSIX regexps; the GNU operators are not special (e.g.,
     '\w' matches a literal 'w').  Interval expressions are allowed.

'--traditional'
     Match traditional Unix 'awk' regexps.  The GNU operators are not
     special.  Because BWK 'awk' supports them, the POSIX character
     classes ('[[:alnum:]]', etc.)  are available.  So too, interval
     expressions are allowed.  Characters described by octal and
     hexadecimal escape sequences are treated literally, even if they
     represent regexp metacharacters.

'--re-interval'
     This option remains for backwards compatibility but no longer has
     any real effect.