Node:Leftmost Longest, Next:Computed Regexps, Previous:Case-sensitivity, Up:Regexp
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).