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How Much Text Matches?

Consider the following:

echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'

This example uses the sub function (which we haven't discussed yet; see String Manipulation Functions) to make a change to the input record. Here, the regexp /a+/ indicates "one or more a characters," and the replacement text is <A>.

The input contains four a characters. awk (and POSIX) regular expressions always match the leftmost, longest sequence of input characters that can match. Thus, all four a characters are replaced with <A> in this example:

$ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
-| <A>bcd

For simple match/no-match tests, this is not so important. But when doing text matching and substitutions with the match, sub, gsub, and gensub functions, it is very important. Understanding this principle is also important for regexp-based record and field splitting (see How Input Is Split into Records, and also see Specifying How Fields Are Separated).