Any awk expression is valid as an awk pattern.
Then the pattern matches if the expression's value is non-zero (if a
number) or non-null (if a string).
The expression is reevaluated each time the rule is tested against a new
input record. If the expression uses fields such as $1, the
value depends directly on the new input record's text; otherwise, it
depends only on what has happened so far in the execution of the
awk program, but that may still be useful.
A very common kind of expression used as a pattern is the comparison expression, using the comparison operators described in section Variable Typing and Comparison Expressions.
Regexp matching and non-matching are also very common expressions.
The left operand of the `~' and `!~' operators is a string.
The right operand is either a constant regular expression enclosed in
slashes (/regexp/), or any expression, whose string value
is used as a dynamic regular expression
(see section Using Dynamic Regexps).
The following example prints the second field of each input record whose first field is precisely `foo'.
$ awk '$1 == "foo" { print $2 }' BBS-list
(There is no output, since there is no BBS site named "foo".) Contrast this with the following regular expression match, which would accept any record with a first field that contains `foo':
$ awk '$1 ~ /foo/ { print $2 }' BBS-list
-| 555-1234
-| 555-6699
-| 555-6480
-| 555-2127
Boolean expressions are also commonly used as patterns. Whether the pattern matches an input record depends on whether its subexpressions match.
For example, the following command prints all records in `BBS-list' that contain both `2400' and `foo'.
$ awk '/2400/ && /foo/' BBS-list -| fooey 555-1234 2400/1200/300 B
The following command prints all records in `BBS-list' that contain either `2400' or `foo', or both.
$ awk '/2400/ || /foo/' BBS-list -| alpo-net 555-3412 2400/1200/300 A -| bites 555-1675 2400/1200/300 A -| fooey 555-1234 2400/1200/300 B -| foot 555-6699 1200/300 B -| macfoo 555-6480 1200/300 A -| sdace 555-3430 2400/1200/300 A -| sabafoo 555-2127 1200/300 C
The following command prints all records in `BBS-list' that do not contain the string `foo'.
$ awk '! /foo/' BBS-list -| aardvark 555-5553 1200/300 B -| alpo-net 555-3412 2400/1200/300 A -| barfly 555-7685 1200/300 A -| bites 555-1675 2400/1200/300 A -| camelot 555-0542 300 C -| core 555-2912 1200/300 C -| sdace 555-3430 2400/1200/300 A
The subexpressions of a boolean operator in a pattern can be constant regular
expressions, comparisons, or any other awk expressions. Range
patterns are not expressions, so they cannot appear inside boolean
patterns. Likewise, the special patterns BEGIN and END,
which never match any input record, are not expressions and cannot
appear inside boolean patterns.
A regexp constant as a pattern is also a special case of an expression
pattern. /foo/ as an expression has the value one if `foo'
appears in the current input record; thus, as a pattern, /foo/
matches any record containing `foo'.
Go to the first, previous, next, last section, table of contents.