Gawk/Regexp-Operator-Details
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3.3.1 Regexp Operators in awk
The escape sequences described
earlier
in Escape Sequences
are valid inside a regexp. They are introduced by a ‘\’ and
are recognized and converted into corresponding real characters as
the very first step in processing regexps.
Here is a list of metacharacters. All characters that are not escape sequences and that are not listed here stand for themselves:
\
This suppresses the special meaning of a character when
matching. For example, ‘\$’
matches the character ‘$’.
^
This matches the beginning of a string. ‘^@chapter’
matches ‘@chapter’ at the beginning of a string,
for example, and can be used
to identify chapter beginnings in Texinfo source files.
The ‘^’ is known as an anchor, because it anchors the pattern to
match only at the beginning of the string.
It is important to realize that ‘^’ does not match the beginning of
a line (the point right after a ‘\n’ newline character) embedded in a string.
The condition is not true in the following example:
if ("line1\nLINE 2" ~ /^L/) …
$
This is similar to ‘^’, but it matches only at the end of a string.
For example, ‘p$’
matches a record that ends with a ‘p’. The ‘$’ is an anchor
and does not match the end of a line
(the point right before a ‘\n’ newline character)
embedded in a string.
The condition in the following example is not true:
if ("line1\nLINE 2" ~ /1$/) …
. (period)
This matches any single character,
including the newline character. For example, ‘.P’
matches any single character followed by a ‘P’ in a string. Using
concatenation, we can make a regular expression such as ‘U.A’, which
matches any three-character sequence that begins with ‘U’ and ends
with ‘A’.
In strict POSIX mode (see section Command-Line Options),
‘.’ does not match the NUL
character, which is a character with all bits equal to zero.
Otherwise, NUL is just another character. Other versions of awk
may not be able to match the NUL character.
[…]
This is called a bracket expression.16
It matches any one of the characters that are enclosed in
the square brackets. For example, ‘[MVX]’ matches any one of
the characters ‘M’, ‘V’, or ‘X’ in a string. A full
discussion of what can be inside the square brackets of a bracket expression
is given in
Using Bracket Expressions.
[^…]
This is a complemented bracket expression. The first character after
the ‘[’ must be a ‘^’. It matches any characters
except those in the square brackets. For example, ‘[^awk]’
matches any character that is not an ‘a’, ‘w’,
or ‘k’.
|
This is the alternation operator and it is used to specify
alternatives. The ‘|’ has the lowest precedence of all the regular
expression operators. For example, ‘^P|[aeiouy]’ matches any string
that matches either ‘^P’ or ‘[aeiouy]’. This means it matches
any string that starts with ‘P’ or contains (anywhere within it)
a lowercase English vowel.
The alternation applies to the largest possible regexps on either side.
(…)
Parentheses are used for grouping in regular expressions, as in
arithmetic. They can be used to concatenate regular expressions
containing the alternation operator, ‘|’. For example,
‘@(samp|code)\{[^}]+\}’ matches both ‘@code{foo}’ and
‘@samp{bar}’.
(These are Texinfo formatting control sequences. The ‘+’ is
explained further on in this list.)
The left or opening parenthesis is always a metacharacter; to match one literally, precede it with a backslash. However, the right or closing parenthesis is only special when paired with a left parenthesis; an unpaired right parenthesis is (silently) treated as a regular character.
*
This symbol means that the preceding regular expression should be
repeated as many times as necessary to find a match. For example, ‘ph*’
applies the ‘*’ symbol to the preceding ‘h’ and looks for matches
of one ‘p’ followed by any number of ‘h’s. This also matches
just ‘p’ if no ‘h’s are present.
There are two subtle points to understand about how ‘*’ works.
First, the ‘*’ applies only to the single preceding regular expression
component (e.g., in ‘ph*’, it applies just to the ‘h’).
To cause ‘*’ to apply to a larger subexpression, use parentheses:
‘(ph)*’ matches ‘ph’, ‘phph’, ‘phphph’, and so on.
Second, ‘*’ finds as many repetitions as possible. If the text
to be matched is ‘phhhhhhhhhhhhhhooey’, ‘ph*’ matches all of
the ‘h’s.
+
This symbol is similar to ‘*’, except that the preceding expression must be
matched at least once. This means that ‘wh+y’
would match ‘why’ and ‘whhy’, but not ‘wy’, whereas
‘wh*y’ would match all three.
?
This symbol is similar to ‘*’, except that the preceding expression can be
matched either once or not at all. For example, ‘fe?d’
matches ‘fed’ and ‘fd’, but nothing else.
{n}
{n,}
{n,m}
One or two numbers inside braces denote an interval expression.
If there is one number in the braces, the preceding regexp is repeated
n times.
If there are two numbers separated by a comma, the preceding regexp is
repeated n to m times.
If there is one number followed by a comma, then the preceding regexp
is repeated at least n times:
wh{3}y- Matches ‘
whhhy’, but not ‘why’ or ‘whhhhy’. wh{3,5}y- Matches ‘
whhhy’, ‘whhhhy’, or ‘whhhhhy’ only. wh{2,}y- Matches ‘
whhy’, ‘whhhy’, and so on.
In regular expressions, the ‘*’, ‘+’, and ‘?’ operators,
as well as the braces ‘{’ and ‘}’,
have
the highest precedence, followed by concatenation, and finally by ‘|’.
As in arithmetic, parentheses can change how operators are grouped.
In POSIX awk and gawk, the ‘*’, ‘+’, and
‘?’ operators stand for themselves when there is nothing in the
regexp that precedes them. For example, /+/ matches a literal
plus sign. However, many other versions of awk treat such a
usage as a syntax error.
Footnotes
(16)
In other literature, you may see a bracket expression referred to as either a character set, a character class, or a character list.
Next: Interval Expressions, Up: Regexp Operators [Contents][Index]
