echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
This example uses the sub function (which we haven't discussed yet;
see String 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 Records,
and also see Field Separators).