in reply to
Mastering Regular Expressions
One nice feature of the book (both editions) should also be mentioned here: Its singularly ingenious typesetting.
Using a sophisticated but quite intuitive notation the author maintains always a clear picture of what portion of an expression or position in a string he is talking about, where exactly literal space characters occur, what matches what, which parentheses belong together, and so on.
This is a significant factor in keeping the discussion of such complex matters concise and straightforward.
I have yet to see other technical writing reaching the same level of clarity through just the right kind of typesetting.