Mastering Regular Expressions really is the seminal text, but I highly suggest reading the O'Reilly Sed/awk book as well. They include full history behind the evolution of the original line editor ed -> sed & g/re/p (etymologically derived from g[lobally-replace]/re[gex]/p[rint]) -> awk. The original vi also derived a lot of it's commands from 'ex' (the underlying editor, based on the original ed).

All history aside, 1: Sed&awk 2: Mastering Regexp 3: The vim book has a great chapter on regular expressions (it's been ~10 years since I've touched it, but I remember being gifted a copy from a colleague of my father when I was in my teens and it helped me grok it)

The glibc programming manual is actually really comprehensive. kernel.org/docs/man-pages/ gives a pretty in depth analysis of POSIX.2 regex. I'm sure Perldocs also have PCRE's very well documented.