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
Variable Typing and Comparison Expressions,
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 Using Dynamic 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 Operator Precedence (How Operators Nest).