Because of this behavior, we say the repetition operators (, ,, and ) are greedy, meaning they match as much as theycan and backtrack from there. If you put a question mark afterthem (, , , ), they become nongreedy and start bymatching as little as possible, matching more only when the remainingpattern does not fit the smaller match.
The upper bound is optional, if omitted any number of occurences equal to or greater than the lower bound is acceptable. The following sample matches two or more consecutive digits.
If that tries to match somelong series of zeros and ones with no trailing b character, thematcher will first go through the inner loop until it runs out ofdigits. Then it notices there is no b, so it backtracks oneposition, goes through the outer loop once, and gives up again, tryingto backtrack out of the inner loop once more. It will continue to tryevery possible route through these two loops. This means the amount ofwork doubles with each additional character. For even just a fewdozen characters, the resulting match will take practically forever.
(5) Modify the class of regular expressions to be like the wild cardsin various shells: matches are implicitly anchored at both ends, matches any number of characters, and matchesany single character.
A dot in the pattern matches all characters, including those indicating newline. Without it, a dot does not match when the current position is at a newline. This option is equivalent to Perl option and it can be changed within a pattern by a option setting. A negative class, such as , always matches newline characters, independent of the setting of this option.
These metacharacters have negated forms. Use to match any character except a word character. Use to match a non-digit character. Use to match anything but whitespace. Use to match anywhere except a word boundary.
Will try to match with if the group with given id or nameexists, and with if it doesn’t. is optional andcan be omitted. For example, is a poor emailmatching pattern, which will match with as well as, but not with .
matches the beginning of the input string $ matches the end of the input string * matches zero or more occurrences of the previous characterThis is quite a useful class; in my own experience of using regularexpressions on a day-to-day basis, it easily accounts for 95 percentof all instances.
If the potential matches in are more than the simplest English words, you will get false positives. also matches punctuation characters, whitespace, and numbers. Be specific! The metacharacter represents all alphanumeric characters () and the underscore:
Regular expressions allow you to group and capture portions of the match for later use. To extract an American telephone number of the form from a string:
Note especially the escaping of the parentheses within . Parentheses are special in Perl 5 regular expressions. They group atoms into larger units and also capture portions of matching strings. To match literal parentheses, escape them with backslashes as seen in .
Perl 5.10 added named captures, which allow you to capture portions of matches from applying a regular expression and access them later, such as finding a phone number in a string of contact information:
Parentheses enclose the capture. The construct names this particular capture and must immediately follow the left parenthesis. The remainder of the capture is a regular expression.
When a match against the enclosing pattern succeeds, Perl stores the portion of the string which matches the enclosed pattern in the magic variable . In this hash, the key is the name of the capture and the value is the appropriate portion of the matched string.
In the book, the regular expression matcher is part of aprogram that mimics grep, but the regular expression code is completelyseparable from its surroundings.