we want to find as many words as possible without having to go back

[suchona.kani.z]

When using lookahead assertions, we use ?=syntax - for example, a pattern A(?=B)simply says "look for A, but only move on if B follows". This is important because it allows us to determine if the expression matches the next characters without having to go back or forward.

In this case. We can rewrite the pattern to include words from \w+to (?=(\w+))\1. This may seem a bit counterintuitive at first glance.

In our rewritten pattern, (?=(\w+))\1we tell the search engine to look for the longest word in the current position. The pattern in the inner brackets, (\w+), tells the search engine to remember the content, and we can \1use to refer to it later.

This solves our problem because we can use the lookahead function denmark consumer email list to w+find the word as a whole and \1reference it with the pattern. Essentially, we can +implement a possessive quantifier that must match the whole word and not parts of it.
Avoid nested quantifiers – for example(a+)*
Avoid ORs with overlapping clauses - for example(b|b)*
Depending on the engine, some regular expressions written with nested quantifiers and overlapping clauses may execute quickly, but there is no guarantee. Better safe than sorry.
In our first example, the given pattern covers the words, but when it encounters an invalid word, the +quantifier forces it to go back until it succeeds or fails. In our rewritten example, we used lookahead to find a valid word that was compared as a whole and \1included in the pattern.

Let's run this new pattern together with our previous quantifiers to see if we have the same problem:


Voila! We see that the regular expression is executed and we get an output immediately; it took about 0.0052 seconds to get a result.


In this blog post, we learned how to prevent regular expressions from being used for denial-of-service attacks. We took a deep dive into how regular expressions work to understand why and how this problem occurs. We then looked at an example of a regular expression pattern with such a vulnerability and showed how to close the holes that attackers can exploit.

You can find more exciting topics from the adesso world in our previously published blog posts .