Comment by js2 | Hacker News Reader

Comment by js2 | Hacker News Reader

Python people tend to always compile their regular expressions; I guess they aren't used to writing throw-away scripts like in Perl

This is so that the RE isn't compiled each time it's used. These days the RE module maintains an internal cache for compiled RE's, but that wasn't always the case.

The 1.5.2 docs for compile() say "using compile() is more efficient when the expression will be used several times in a single program" while the 2.7.2 docs add the note that "the compiled versions of the most recent patterns passed to re.match(), re.search() or re.compile() are cached, so programs that use only a few regular expressions at a time needn’t worry about compiling regular expressions."

Edit: apparently the cache was present in 1.5.2; it just wasn't documented. In 1.5.2 the cache size was 20. In 2.0 it was increased to 100.

Still: explicit is better than implicit.

As well, I find assigning the RE to a well-named variable tends to make for more readable code. RE patterns can be horribly ugly and I like to get their definitions out of the way of the logic using them.

The point being, though, that perl regexes are compiled at parse time, with no extra syntax hanging around.

And no, I simply have to disagree that a separate object instantiation is "more readable" than the straightforward syntax describing the object itself. That this argument should appear in a discussion of a dynamically bound scripting language shows as well as anything why the python community is eating itself. A LISP or Ruby nut would never have made that claim.

The point to having code there is to read it. Hiding it doesn't make it more readable, it just fools you into thinking you understand it better than you do. You probably call your regexes stuff like "split_at_word_boundaries", right? And how many bugs have you had to deal with because the whitespace conventions aren't clear from your source?

> RE patterns can be horribly ugly

It worth to be noted that the verbose flag (re.X or re.VERBOSE in Python) may help to get more readable/friendly regexes (by allowing whitespace and comments).

I'm desperately fighting the urge to snark: but another very effective treatment for dealing with unreadable regex syntax is to learn regex syntax. In my experience probably 90% of people who complain about regular expressions simply don't know them very well.

It's a declarative language without symbols or recursion: that means that it's just not well-suited to being "broken down and simplified". You just have to bite the bullet and learn it. With a little practice, the basic features (e.g. beginning/end of line markers, character classes and captures, maybe non-capturing parentheses too) should be readable without trouble.

I think you missed my point.

I agree that for trivial regex it's useless. For instance (directly taken from Python doc) in this case:

  a = re.compile(r"""\d +  # the integral part
                     \.    # the decimal point
                     \d *  # some fractional digits""", re.X)
  b = re.compile(r"\d+\.\d*")

that's useless. But for more complex problems it may be useful (e.g. http://www.doughellmann.com/PyMOTW/re/ I know email matching regexes are not really complex or even a good example of use of regexes but that's the first example I found).

Another point is that even if I can read/write (that's the case) regexes other people may have to deal with my code and it's a well known fact that many people doesn't understand/like regex, so splitting them in small "chunks" may help them.

To be fair: my argument wasn't against the use of the /x suffix to a regex. I'm sure that there are circumstances in the wild (though quite honestly I can't recall seeing any in production code, and I've written and read a truckload of regexes over the years) where it's used productively to document a really hairy expression.

I'm just saying that 90% of the time when users complain about not being able to read a regex, the solution should be "hit the books" not "rewrite the expression".

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