3.5 How Much Text Matches?

Consider the following:

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

This example uses the sub() function to make a change to the input record. (sub() replaces the first instance of any text matched by the first argument with the string provided as the second argument; see section String-Manipulation Functions.) 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 section How Input Is Split into Records, and also see section Specifying How Fields Are Separated).