(gawk.info.gz) Expression Patterns

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(gawk.info.gz) Regexp Patterns

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(gawk.info.gz) Ranges

 
 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 ( Computed 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, because there is no BBS site with the exact name
 `foo'.)  Contrast this with the following regular expression match,
 which accepts any record with a first field that contains `foo':
 
      $ awk '$1 ~ /foo/ { print $2 }' BBS-list
      -| 555-1234
      -| 555-6699
      -| 555-6480
      -| 555-2127
 
    A regexp constant as a pattern is also a special case of an
 expression pattern.  The expression `/foo/' has the value one if `foo'
 appears in the current input record. Thus, as a pattern, `/foo/'
 matches any record containing `foo'.
 
    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
 `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, of course):
 
      $ 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', `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 which can appear in
 patterns is described in  Precedence.
 

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(gawk.info.gz) Regexp Patterns

(gawk.info.gz) Pattern Overview

(gawk.info.gz) Ranges