CODEWITHSUNDEEP
phpregexpcreregex-lookarounds
crazy_in_love
crazy_in_love

Reputation: 145

Lookahead, lookbehind condition in regular expression

The following example is about using lookahead assertion as a condition. I found it in the PHP manual at: http://www.php.net/manual/en/regexp.reference.conditional.php

(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

Here's the description about this regex:

The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.

Could anyone tell me why we use lookahead assertion as the condition in this example? Why don't we use lookbehind assertion? I get confused when they're used as conditions like this because I don't know how do they match the subject string. Thanks in advance!

Upvotes: 0

Views: 743

Answers (1)

Gareth
Gareth

Reputation: 5719

In this case we're using a lookahead assertion to decide which regex to use. It looks like it's deciding between matching dates of the form 01-Jan-12 and 01-01-12. The lookahead assertion sees if there are any letters within what we're trying to match and if so uses the \d{2}-[a-z]{3}-\d{2} to try and match 01-Jan-12 if not it uses \d{2}-\d{2}-\d{2} to try and match 01-01-12.

Upvotes: 1

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