(gawk.info) Regexp Usage

 
 How to Use Regular Expressions
 ==============================
 
    A regular expression can be used as a pattern by enclosing it in
 slashes.  Then the regular expression is tested against the entire text
 of each record.  (Normally, it only needs to match some part of the
 text in order to succeed.)  For example, this prints the second field
 of each record that contains the three characters `foo' anywhere in it:
 
      $ awk '/foo/ { print $2 }' BBS-list
      -| 555-1234
      -| 555-6699
      -| 555-6480
      -| 555-2127
 
    Regular expressions can also be used in matching expressions.  These
 expressions allow you to specify the string to match against; it need
 not be the entire current input record.  The two operators, `~' and
 `!~', perform regular expression comparisons.  Expressions using these
 operators can be used as patterns or in `if', `while', `for', and `do'
 statements.  ( Control Statements in Actions Statements.)
 
 `EXP ~ /REGEXP/'
      This is true if the expression EXP (taken as a string) is matched
      by REGEXP.  The following example matches, or selects, all input
      records with the upper-case letter `J' somewhere in the first
      field:
 
           $ awk '$1 ~ /J/' inventory-shipped
           -| Jan  13  25  15 115
           -| Jun  31  42  75 492
           -| Jul  24  34  67 436
           -| Jan  21  36  64 620
 
      So does this:
 
           awk '{ if ($1 ~ /J/) print }' inventory-shipped
 
 `EXP !~ /REGEXP/'
      This is true if the expression EXP (taken as a character string)
      is _not_ matched by REGEXP.  The following example matches, or
      selects, all input records whose first field _does not_ contain
      the upper-case letter `J':
 
           $ awk '$1 !~ /J/' inventory-shipped
           -| Feb  15  32  24 226
           -| Mar  15  24  34 228
           -| Apr  31  52  63 420
           -| May  16  34  29 208
           ...
 
    When a regexp is written enclosed in slashes, like `/foo/', we call
 it a "regexp constant", much like `5.27' is a numeric constant, and
 `"foo"' is a string constant.
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(gawk.info) Regexp
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