home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

Book HomeLearning Perl, 3rd EditionSearch this book

8.2. General Quantifiers

A quantifier in a pattern means to repeat the preceding item a certain number of times. We've already seen three quantifiers: *, +, and ?. But if none of those three suits your needs, just use a comma-separated pair of numbers inside curly braces ({}) to specify exactly how few and how many repetitions are allowed.

So the pattern /a{5,15}/ will match from five to fifteen repetitions of the letter a. If the a appears three times, that's too few, so it won't match. If it appears five times, it's a match. If it appears ten times, that's still a match. If it appears twenty times, just the first fifteen will match, since that's the upper limit.

If you omit the second number (but include the comma), there's no upper limit to the number of times the item will match. So, /(fred){3,}/ will match if there are three or more instances of fred in a row (with no extra characters, like spaces, allowed between each fred and the next). There's no upper limit, so that would match eighty-eight instances of fred, if you had a string with that many.

If you omit the comma as well as the upper bound, the number given is an exact count: /\w{8}/ will match exactly eight word characters (occuring as part of a larger string, perhaps).

In fact, the three quantifier characters that we saw earlier are just common shortcuts. The star is the same as the quantifier {0,}, meaning zero or more. The plus is the same as {1,}, meaning one or more. And the question mark could be written as {0,1}. In practice, it's unusual to need any curly-brace quantifiers, since the three shortcut characters are nearly always the only ones needed.

Library Navigation Links

Copyright © 2002 O'Reilly & Associates. All rights reserved.