(gawk.info.gz) Regexp Usage

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(gawk.info.gz) Regexp

(gawk.info.gz) Regexp

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 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, the following prints the
 second field of each record that contains the string `foo' anywhere in
 it:
 
      $ awk '/foo/ { print $2 }' BBS-list
      -| 555-1234
      -| 555-6699
      -| 555-6480
      -| 555-2127
 
    `~' (tilde), `~' operator 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.)  For example:
 
      EXP ~ /REGEXP/
 
 is true if the expression EXP (taken as a string) matches REGEXP.  The
 following example matches, or selects, all input records with the
 uppercase 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
 
    This next example is true if the expression EXP (taken as a
 character string) does _not_ match REGEXP:
 
      EXP !~ /REGEXP/
 
    The following example matches, or selects, all input records whose
 first field _does not_ contain the uppercase 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 enclosed in slashes, such as `/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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