Using the Regular Expressions Tool



For those of you who, like me, make good use of the regular expression tool at, you should be aware of the regex engines that it uses:

How close does regex101 emulates the engines?

For PCRE , regex101 compiles its library from PCRE 16-bit library, while PHP uses PCRE 8-bit library, so there are discrepancies in non-UTF mode (see the question below). Other discrepancies are caused by differences in regex101's and PHP's implementation of matching and replacement functions.

Known issue:

For JavaScript , regex101 uses the browser's regex engine, so you may see different results on different browsers for the same test case.

For Python , regex101 implements it on top of PCRE library, by suppressing features not available in Python. However, there are minor differences which have not been ironed out.

Check the list for known issues.

One other note is that you should always set the unicode flag when using, else results may not match the results of the KM Search using Regular Expression action.