[Biganon]

Yes, two issues are a) knowing what to escape and b) what is supported or not. I was weirdly surprised to discover that there are some useful functionalities that are basically supported nowhere... except in vim. I think it was variable-length lookbehinds but don't quote me on this.

[ygra]

Lookbehind is restricted in many regex engines that support it (many don't at all). Some require the lookbehind to be constant-length (so if you have alternatives in there, they all have to have the same length, and you can't use quantifiers, basically). Some require it to be finite-but-known-ahead-of-time-length, so something like (?<=a{3,6}) is okay, but (?<=a{3,) is not. Also (?<=a|bb) would be okay.

.NET's System.Text.RegularExpressions.Regex is one implementation that has no restrictions on lookbehind. Having used PowerShell for so long it now happens sometimes that I forget when writing regexes for other implementations, as it's really convenient at times.

[asicsp]

yeah, typically you would get error if you use variable length lookbehinds

a few do support it (for ex: Python's 3rd party regex module) and sometimes you could workaround with \K (similar to \zs in vim) [1]

And there are other frustrating differences between implementations, for example \g definition is very different between Ruby and Python, character set operations are not found everywhere, etc. Plus, BRE/ERE versions found in command line tools do not even support features like non-greedy and lookaround

[1] https://stackoverflow.com/questions/11640447/variable-length...