How to Use Regular Expressions

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, this prints the second field of each record that contains the three characters `foo' anywhere in it:

$ awk '/foo/ { print $2 }' BBS-list
-| 555-1234
-| 555-6699
-| 555-6480
-| 555-2127

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.

exp ~ /regexp/

This is true if the expression exp (taken as a string) is matched by regexp. The following example matches, or selects, all input records with the upper-case 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

exp !~ /regexp/

This is true if the expression exp (taken as a character string) is not matched by regexp. The following example matches, or selects, all input records whose first field does not contain the upper-case 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 written enclosed in slashes, like /foo/, we call it a regexp constant, much like 5.27 is a numeric constant, and "foo" is a string constant.

Go to the first, previous, next, last section, table of contents.