Regular Expression Lookbehind doesn't work with quantifiers ('+' or '*')

42
votes

I am trying to use lookbehinds in a regular expression and it doesn't seem to work as I expected. So, this is not my real usage, but to simplify I will put an example. Imagine I want to match "example" on a string that says "this is an example". So, according to my understanding of lookbehinds this should work:

(?<=this\sis\san\s*?)example

What this should do is find "this is an", then space characters and finally match the word "example". Now, it doesn't work and I don't understand why, is it impossible to use '+' or '*' inside lookbehinds?

I also tried those two and they work correctly, but don't fulfill my needs:

(?<=this\sis\san\s)example
this\sis\san\s*?example

I am using this site to test my regular expressions: http://gskinner.com/RegExr/

5
regexlookbehind
This needs a tag that identifies the language or environment where you use them. .NET's regular expressions handle this without a problem. - Joey
Notice! If your regex would work like you want it will also match example from this: this is anexample. So if you don't want that you should remove the ? - noob
micha: They should probably just change the * to a +. Removing the ? has no effect in that regard. But indeed, *? as a quantifier is useless and unnecessary in this case as there isn't any more whitespace to match after that, so \s*? is equivalent to \s*. - Joey

5 Answers

41
votes

Many regular expression libraries do only allow strict expressions to be used in look behind assertions like:

  • only match strings of the same fixed length: (?<=foo|bar|\s,\s) (three characters each)
  • only match strings of fixed lengths: (?<=foobar|\r\n) (each branch with fixed length)
  • only match strings with a upper bound length: (?<=\s{,4}) (up to four repetitions)

The reason for these limitations are mainly because those libraries can’t process regular expressions backwards at all or only a limited subset.

Another reason could be to avoid authors to build too complex regular expressions that are heavy to process as they have a so called pathological behavior (see also ReDoS).

See also section about limitations of look-behind assertions on Regular-Expressions.info.

16
votes

Hey if your not using python variable look behind assertion you can trick the regex engine by escaping the match and starting over by using \K.

This site explains it well .. http://www.phpfreaks.com/blog/pcre-regex-spotlight-k ..

But pretty much when you have an expression that you match and you want to get everything behind it using \K will force it to start over again...

Example:

string = '<a this is a tag> with some information <div this is another tag > LOOK FOR ME </div>'

matching /(\<a).+?(\<div).+?(\>)\K.+?(?=\<div)/ will cause the regex to restart after you match the ending div tag so the regex won't include that in the result. The (?=\div) will make the engine get everything in front of ending div tag

4
votes

What Amber said is true, but you can work around it with another approach: A non-capturing parentheses group

(?<=this\sis\san)(?:\s*)example

That make it a fixed length look behind, so it should work.

0
votes

Most regex engines don't support variable-length expressions for lookbehind assertions.

0
votes

You can use sub-expressions.

(this\sis\san\s*?)(example)

So to retrieve group 2, "example", $2 for regex, or \2 if you're using a format string (like for python's re.sub)