Help, Search and Substitute

[Simon Brunning]

> From:	andy47 at my-deja.com [SMTP:andy47 at my-deja.com]
> > # based on re-example-5.py
> > # from (the eff-bot guide to) The Python Standard Library
> > # http://www.pythonware.com/people/fredrik/librarybook.htm
> >
> > import re
> > import string
> >
> > symbol_map = { "foo": "FOO", "bar": "BAR" }
> >
> > def symbol_replace(match, get=symbol_map.get):
> >     return get(match.group(1), "")
> >
> > symbol_pattern = re.compile(
> >     "\{(" + string.join(map(re.escape, symbol_map.keys()), "|") + ")
> \}"
> >     )
> >
> > print symbol_pattern.sub(symbol_replace, "{foo}fie{bar}")
> >
> > </F>
> 
> Folks,
> 
> I'm probably being really dumb here. I can see that the above works,
> and I've got a vague idea of how it works but the detail is escaping
> me. Can anyone explain how it works in words of one syllable to us
> newbies? In particular, I'm a little lost as to how the function
> symbol_replace works.
 
/F sent me a similar snippet a while back, in response to a similar query,
and it took me a little while to understand it. Here goes...

Firstly, symbol_map is a dictionary containing the 'from' & 'to' values.
(Let's ignore the symbol_replace function until later...)

/F then builds a re, basically consisting of all the 'from' values, joined
by '|'s. (Note the use if re.escape - this escapes any characters having
special meaning within regular expressions, such as '|' or '['. Have a play
with it interactively.)

/F then calls symbol_pattern's sub method. This takes two arguments, the
second being the string to be changed. The first argument of the sub method
can be one of two things - it can simply be a string, in which case all
substrings matching the re have that string substituted. Or, if you need
something a little more complex, as you do, it can be a function...

This is where the symbol_replace function comes in. It has been passed the
match value, i.e. the substring that the re matched, so it can use that as a
key value to look up the 'to' value in the symbol_map dictionary. It returns
this to the sub method, which uses it to do the replacement.

(I don't know why /F made 'get' a defaulted argument of the function - but
then he's *far* cleverer that I, so there was *bound* to be something that I
didn't understand!)

Cheers,
Simon Brunning
TriSystems Ltd.
sbrunning at trisystems.co.uk
When I was a kid, I used to pray every night for a new bicycle. Then I
realised that the Lord, in his wisdom, didn't work that way. So I just stole
one and asked him to forgive me. - Emo Philips 




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