gawk-Specific Regexp Operators
GNU software that deals with regular expressions provides a number of
additional regexp operators. These operators are described in this
section and are specific to
they are not available in other
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 (‘_’):
Matches any whitespace character. Think of it as shorthand for ‘[[:space:]]’.
Matches any character that is not whitespace. Think of it as shorthand for ‘[^[:space:]]’.
Matches any word-constituent character—that is, it matches any letter, digit, or underscore. Think of it as shorthand for ‘[[:alnum:]_]’.
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.
/\<away/ matches ‘away’ but not
Matches the empty string at the end of a word.
/stow\>/ matches ‘stow’ but not ‘stowaway’.
Matches the empty string at either the beginning or the end of a word (i.e., the word boundary). For example, ‘\yballs?\y’ matches either ‘ball’ or ‘balls’, as a separate word.
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. For other programs,
gawk’s regexp library routines 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
awk. They are provided for compatibility with other
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
gawk interprets characters in regexps:
In the default case,
gawk provides all the facilities of
POSIX regexps and the
GNU regexp operators.
GNU regexp operators described
in Regexp Operators.
Only POSIX regexps are supported; the GNU operators are not special (e.g., ‘\w’ matches a literal ‘w’). Interval expressions are allowed.
awk regexps are matched. The GNU operators
are not special, and interval expressions are not available.
The POSIX character classes (‘[[:alnum:]]’, etc.) are supported,
as Brian Kernighan’s
awk does support them.
Characters described by octal and hexadecimal escape sequences are
treated literally, even if they represent regexp metacharacters.
Allow interval expressions in regexps, if --traditional has been provided. Otherwise, interval expressions are available by default.