File: gawk.info,  Node: Pattern Overview,  Next: Using Shell Variables,  Up: Patterns and Actions

7.1 Pattern Elements
====================

* Menu:

* Regexp Patterns::             Using regexps as patterns.
* Expression Patterns::         Any expression can be used as a pattern.
* Ranges::                      Pairs of patterns specify record ranges.
* BEGIN/END::                   Specifying initialization and cleanup rules.
* BEGINFILE/ENDFILE::           Two special patterns for advanced control.
* Empty::                       The empty pattern, which matches every record.

Patterns in 'awk' control the execution of rules--a rule is executed
when its pattern matches the current input record.  The following is a
summary of the types of 'awk' patterns:

'/REGULAR EXPRESSION/'
     A regular expression.  It matches when the text of the input record
     fits the regular expression.  (*Note Regexp::.)

'EXPRESSION'
     A single expression.  It matches when its value is nonzero (if a
     number) or non-null (if a string).  (*Note Expression Patterns::.)

'BEGPAT, ENDPAT'
     A pair of patterns separated by a comma, specifying a "range" of
     records.  The range includes both the initial record that matches
     BEGPAT and the final record that matches ENDPAT.  (*Note Ranges::.)

'BEGIN'
'END'
     Special patterns for you to supply startup or cleanup actions for
     your 'awk' program.  (*Note BEGIN/END::.)

'BEGINFILE'
'ENDFILE'
     Special patterns for you to supply startup or cleanup actions to be
     done on a per-file basis.  (*Note BEGINFILE/ENDFILE::.)

'EMPTY'
     The empty pattern matches every input record.  (*Note Empty::.)