gawk: GNU Regexp Operators
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 whitespace character. Think of it as shorthand for
'[[:space:]]'.
'\S'
Matches any character that is not whitespace. 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,
'/\<away/' matches 'away' but not 'stowaway'.
'\>'
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'.
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 (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 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, and interval expressions are not available. Because BWK
'awk' supports them, the POSIX character classes ('[[:alnum:]]',
etc.) are available. Characters described by octal and
hexadecimal escape sequences are treated literally, even if they
represent regexp metacharacters.
'--re-interval'
Allow interval expressions in regexps, if '--traditional' has been
provided. Otherwise, interval expressions are available by
default.