A regular expression can be used as a pattern by enclosing it in slashes. Then the regular expression is tested against the entire text of each record. (Normally, it only needs to match some part of the text in order to succeed.) For example, the following prints the second field of each record where the string ‘li’ appears anywhere in the record:
$ awk '/li/ { print $2 }' mail-list
-| 555-5553
-| 555-0542
-| 555-6699
-| 555-3430
Regular expressions can also be used in matching expressions. These
expressions allow you to specify the string to match against; it need
not be the entire current input record. The two operators ‘~’
and ‘!~’ perform regular expression comparisons. Expressions
using these operators can be used as patterns, or in if,
while, for, and do statements.
(See Control Statements in Actions.)
For example, the following is true if the expression exp (taken
as a string) matches regexp:
exp ~ /regexp/
This example matches, or selects, all input records with the uppercase letter ‘J’ somewhere in the first field:
$ awk '$1 ~ /J/' inventory-shipped -| Jan 13 25 15 115 -| Jun 31 42 75 492 -| Jul 24 34 67 436 -| Jan 21 36 64 620
So does this:
awk '{ if ($1 ~ /J/) print }' inventory-shipped
This next example is true if the expression exp (taken as a character string) does not match regexp:
exp !~ /regexp/
The following example matches, or selects, all input records whose first field does not contain the uppercase letter ‘J’:
$ awk '$1 !~ /J/' inventory-shipped -| Feb 15 32 24 226 -| Mar 15 24 34 228 -| Apr 31 52 63 420 -| May 16 34 29 208 ...
When a regexp is enclosed in slashes, such as /foo/, we call it
a regexp constant, much like 5.27 is a numeric constant and
"foo" is a string constant.