Pattern Overview (The GNU Awk User’s Guide)

7.1 Pattern Elements

• 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. (See section Regular Expressions.)
expression: A single expression. It matches when its value is nonzero (if a number) or non-null (if a string). (See section Expressions as 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. (See section Specifying Record Ranges with Patterns.)
BEGIN

END

Special patterns for you to supply startup or cleanup actions for your awk program. (See section The BEGIN and END Special Patterns.)
BEGINFILE

ENDFILE

Special patterns for you to supply startup or cleanup actions to be done on a per-file basis. (See section The BEGINFILE and ENDFILE Special Patterns.)
empty: The empty pattern matches every input record. (See section The Empty Pattern.)