Perl in a Nutshell, 2nd Edition

Quantifiers

Quantifiers are used to specify the number of instances of the previous element that can match. For instance, you could say “match any number of a’s, including none” (a*), or “match between 5 and 10 instances of the word `owie’ ((owie){5,10})”.

Quantifiers, by nature, are greedy. That is, the way the Perl regular expression “engine” works is that it will look for the biggest match possible (the farthest to the right) unless you tell it not to. Say you are searching a string that reads:

a whatever foo, b whatever foo

and you want to find a and foo with something in between. You might use:

/a.*foo/

A . followed by a * looks for any character, any number of times, until foo is found. But since Perl will look as far to the right as possible to find foo, the first instance of foo is swallowed up by the greedy .* expression.

Therefore, all the quantifiers have a notation that allows for minimal matching, so they are nongreedy. This notation uses a question mark immediately following the quantifier to force Perl to look for the earliest available match (farthest to the left). The following table lists the regular expression quantifiers and their nongreedy forms:

Maximal	Minimal	Allowed range
{n,m}	{n,m}?	Must occur at least n times but no more than m times
{n,}	{n,}?	Must occur at least n times
{n}	{n}?	Must match exactly n times
*	*?	0 or more times (same as {0,})
+	+?	1 or more times (same as {1,})
?	??	0 or 1 time (same as {0,1})