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/
Introduction to the JavaScript regex lookbehind In regular expressions, a lookbehind matches an element if there is another specific element before it. A lookbehind has the following syntax: (?<=Y)X. In this syntax, the pattern match X if there is Y before it.
The good news is that you can use lookbehind anywhere in the regex, not only at the start. If you want to find a word not ending with an “s”, you could use \b\w+(? <! s)\b.
quantifier matches the preceding element one or more times, but as few times as possible. It is the lazy counterpart of the greedy quantifier + . For example, the regular expression \b\w+?\ b matches one or more characters separated by word boundaries.
JavaScript doesn't support any lookbehind, but it can support lookaheads.
Many regular expression libraries do only allow strict expressions to be used in look behind assertions like:
(?<=foo|bar|\s,\s)
(three characters each)(?<=foobar|\r\n)
(each branch with fixed 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.
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
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