It have been a few years since I read the code, but it was so beautifully written that I cannot imagine it was screwed up in the meantime: Edi Weitz' CL-PPCRE[1] is a beautiful implementation in CL and highly recommended if one understands one aspect (CL or Perl compatible regular expressions) and wants to learn the other one. IIRC he even discovered some bugs in the original perl implementation while creating this library.

[1] https://github.com/edicl/cl-ppcre