Hacker News Reader

Comment by blhack | Hacker News Reader

On UNIX/Linux it is also viewed by many as a modern replacement for Perl

I still have a LOT of perl in use simply because of my hatred of python's regular expressions and I suspect that I'm not the only one...

View on HN

I dunno man, my way of thinking is that if you're not screen scraping, regexes shouldn't show up all that much and if they're not showing up that often, Python's way of doing it isn't so bad.

Not much of a condolence if you're screen scraping, but still.

what do you hate about python regexes?

It's difficult to explain what it is, precisely, about python RegExes that make them awkward, and perhaps if I had never encountered the syntatic regex sugar that is perl, I would have never noticed. But, as a total python advocate and frequent writer of python scripts - I still find my brain hitting wait-states that don't occur in perl when I need to pattern match. I can live with that because of the 50+ things that make python so much better (for me) than Perl (Hash of Array / Array of Hash at the top of the list) - but it's sad that I still don't regex in Python as fluidly as perl.

I think the first hint that Python wasn't developed with RegEx users in mind would be that in the 700 Page "Learning Python, 3rd Edition" - Regular expressions aren't even mentioned in the Index.

well, I don't disagree that there's a tradeoff there - the s/ and =~ syntaxes are super neat, as opposed to the readable and consistent python style.

I can see that as a reason for preferring perl's syntax, but I just don't understand hating python's regex, which is what my question was about.

It probably has a lot to do with my lack of programming knowledge (I've never taken any formal programming classes [err...I took java in college but never actually went to the scheduled class because it was a lot of covering what these cRaZy things called variables were])...

A replacement on python looks like this...

foo = re.compile("<") bar = foo.replace(">", bar)

this works, but it just feels less awkward to me than

$bar = s/</>/

hmmm, I would write the equivalent to your perl as:

    >>> re.sub("<", "&lt;", "something < other")
    'something &lt; other'

which seems much more readable and consistent to me than s/a/b, though the s/ syntax is great for vim users like me.

I don't think this has as much to do with your programming knowledge as your python knowledge?

I disagree. Python regexen are a royal pain compared to how they work in Perl. You know what would be pretty funny is if a rogue group of guerilla programmers hacked Perl style regex match and replace syntax into Python.

Heheheh, boy would that get some bees in some bonnets on the Python dev ML. :)

It's been done, by Andrew Dalke, using a Python extension toolkit he developed:

http://dalkescientific.com/Python/python4ply-tutorial.html#p...

He presented it at PyCon a couple years ago, response seemed to be "that's cool".

One perl nicety is that the VM will optimize to only compile the expression once. Python's re.sub() has to compile the expression every time, since it's just a string argument rather than a language construct. You can manually cache the result of re.compile() -- it's just not as sugary that way.

That's actually untrue. If you look in the implementation of the `re` module you would see that it caches the compilation of any regular expression.

Aha -- I almost suggested that they could do some memoization, and indeed they are doing just that. The last 100 unique patterns are compiled and stored in a dictionary for future use. You could still thrash badly if your regex "working set" is more than 100 patterns, but for most people it's probably Good Enough. (Plus you could override re._MAXCACHE if you really want.)

Thanks for pointing out my error, and getting me to read the actual code instead of speculating. :)

More likely: s = "something < other" s = s.replace("<", "<") Even more likely: import cgi s = cgi.escape(s)

In Python, much string manipulation is done with builtins (especially string methods) and library functions. The latter work especially well when you target a specific domain (such as HTML) and will often be implemented in regexps. Writing one's own regexps are for when you actually need a small custom state machine. Many problems are too small or too large to justify the effort.

Reply on news.ycombinator.com