regex - Regular expression using negative lookbehind not working in Notepad++

Question

I have a source file with literally hundreds of occurrences of strings flecha.jpg and flecha1.jpg, but I need to find occurrences of any other .jpg image (i.e. casa.jpg, moto.jpg, whatever)

I have tried using a regular expression with negative lookbehind, like this:

(?<!flecha|flecha1).jpg

but it doesn't work! Notepad++ simply says that it is an invalid regular expression.

I have tried the regex elsewhere and it works, here is an example so I guess it is a problem with NPP's handling of regexes or with the syntax of lookbehinds/lookaheads.

So how could I achieve the same regex result in NPP?

If useful, I am using Notepad++ version 6.3 Unicode

As an extra, if you are so kind, what would be the syntax to achieve the same thing but with optional numbers (in this case only '1') as a suffix of my string? (even if it doesn't work in NPP, just to know)...

I tried (?<!flecha[1]?).jpg but it doesn't work. It should work the same as the other regex, see here (RegExr)

Answer 1

Notepad++ seems to not have implemented variable-length look-behinds (this happens with some tools). A workaround is to use more than one fixed-length look-behind:

(?<!flecha)(?<!flecha1).jpg

As you can check, the matches are the same. But this works with npp.

Notice I escaped the ., since you are trying to match extensions, what you want is the literal .. The way you had, it was a wildcard - could be any character.

About the extra question, unfortunately, as we can't have variable-length look-behinds, it is not possible to have optional suffixes (numbers) without having multiple look-behinds.