How to not match a string with regular expressions

Regular expressions are a robust method for matching strings, and finding patterns in text. But it takes some time to learn, for instance, consider extracting all strings in quotations from this text:

a "sample" text "fragment".

The initial attempt is something like this:

".*"

But since regular expressions use greedy matching, it will match:

"sample" text "fragment"

The fix for this would be to change any character (.) to match any character except " ([^"]):

"[^"]*"

And this solution works, but what if the delimiter is a string and not just a single character? For example:

a <!--sample--> text <!--fragment-->.

You cannot use any character except, you need a any character except one that is followed by, and this is called negative lookahead.

To match any character except one that is followed by --> you can use the following expression:

(.(?!-->))*

Applying this on the sample text matches all text except the e and t in sample and fragment respectively. If we want the entire inner text matched, we need to add the excluded character:

(.(?!-->))*.

This will match the entire text, so now we can apply the delimiters:

<!--(.(?!-->))*.-->

Derek Slager has an online regular expression tester.