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7.1 Pattern Elements

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 Regexp.)
expression
A single expression. It matches when its value is nonzero (if a number) or non-null (if a string). (See Expression Patterns.)
pat1, pat2
A pair of patterns separated by a comma, specifying a range of records. The range includes both the initial record that matches pat1 and the final record that matches pat2. (See Ranges.)
BEGIN
END
Special patterns for you to supply startup or cleanup actions for your awk program. (See BEGIN/END.)
BEGINFILE
ENDFILE
Special patterns for you to supply startup or cleanup actions to be done on a per file basis. (See BEGINFILE/ENDFILE.)
empty
The empty pattern matches every input record. (See Empty.)