I tried to keep it generic, so most of the regex in the book will work in Perl, Ruby, Python, and the libraries since they all originate from Perl's ideas of PCRE. The choice of using Python was mostly because people had read my other book and probably already had Python.
Also lots of engines have the verbose form, problem is there's been too many Perl hackers writing those god awful huge regex so everyone thinks dense and succinct is the only way to write regex.
Perhaps a footnote could mention that this style really is possible in modern regexp implementations? I see chapter 20 is inked in to cover verbose regexps with comments (which is great!), but the current text could give the impression that it's a purely imaginary feature.
I think some examples of actual strings that the RegEx (in section 0.2) actually does match would be quite instructive.
Also, I think word problems develop an essential skill, namely mapping new problems (described usually in natural language) to mathematical concepts using the symbols. Problems come first, the neat symbolic form comes way after and generally the main problem is mapping a vague problem description to a concrete description using the symbolic language provided by mathematical notation.
Also lots of engines have the verbose form, problem is there's been too many Perl hackers writing those god awful huge regex so everyone thinks dense and succinct is the only way to write regex.