7.1.2 Expressions as Patterns

Any awk expression is valid as an awk pattern. The pattern matches if the expression’s value is nonzero (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 on only what has happened so far in the execution of the awk program.

Comparison expressions, using the comparison operators described in Typing and Comparison, are a very common kind of pattern. Regexp matching and nonmatching 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 Computed Regexps). The following example prints the second field of each input record whose first field is precisely ‘li’:

$ awk '$1 == "li" { print $2 }' mail-list

(There is no output, because there is no person with the exact name ‘li’.) Contrast this with the following regular expression match, which accepts any record with a first field that contains ‘li’:

$ awk '$1 ~ /li/ { print $2 }' mail-list
-| 555-5553
-| 555-6699

A regexp constant as a pattern is also a special case of an expression pattern. The expression /li/ has the value one if ‘li’ appears in the current input record. Thus, as a pattern, /li/ matches any record containing ‘li’.

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 the records in mail-list that contain both ‘edu’ and ‘li’:

$ awk '/edu/ && /li/' mail-list
-| Samuel       555-3430     samuel.lanceolis@shu.edu        A

The following command prints all records in mail-list that contain either ‘edu’ or ‘li’ (or both, of course):

$ awk '/edu/ || /li/' mail-list
-| Amelia       555-5553     amelia.zodiacusque@gmail.com    F
-| Broderick    555-0542     broderick.aliquotiens@yahoo.com R
-| Fabius       555-1234     fabius.undevicesimus@ucb.edu    F
-| Julie        555-6699     julie.perscrutabor@skeeve.com   F
-| Samuel       555-3430     samuel.lanceolis@shu.edu        A
-| Jean-Paul    555-2127     jeanpaul.campanorum@nyu.edu     R

The following command prints all records in mail-list that do not contain the string ‘li’:

$ awk '! /li/' mail-list
-| Anthony      555-3412     anthony.asserturo@hotmail.com   A
-| Becky        555-7685     becky.algebrarum@gmail.com      A
-| Bill         555-1675     bill.drowning@hotmail.com       A
-| Camilla      555-2912     camilla.infusarum@skynet.be     R
-| Fabius       555-1234     fabius.undevicesimus@ucb.edu    F
-| Martin       555-6480     martin.codicibus@hotmail.com    A
-| Jean-Paul    555-2127     jeanpaul.campanorum@nyu.edu     R

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, END, BEGINFILE, and ENDFILE, which never match any input record, are not expressions and cannot appear inside Boolean patterns.

The precedence of the different operators that can appear in patterns is described in Precedence.