gawk: Expression Patterns
7.1.2 Expressions as Patterns
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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 (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.
Info Catalog
gawk: Regexp Patterns
gawk: Pattern Overview
gawk: Ranges