Consider the following example:
echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
This example uses the sub function (which we haven't discussed yet,
see section Built-in Functions for String Manipulation)
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. What will the output be?
In other words, how many is "one or more"---will awk match two,
three, or all four `a' characters?
The answer is, awk (and POSIX) regular expressions always match
the leftmost, longest sequence of input characters that can
match. Thus, in this example, all four `a' characters are
replaced with `<A>'.
$ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
-| <A>bcd
For simple match/no-match tests, this is not so important. But when doing
regexp-based field and record splitting, and
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).
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