Backchecking on python regex

Question

I'm trying to write a regular expression match

I'd like to match c99 in files, so long as its not part of a hexadecimal color code for example

Do NOT match on #000c99
DO match on /mysite.com/c99.php
DO match on %20c99.php
DO match on c99?hi=moo

Is this even possible with regex?

@Joel Cornett - I believe they work for fixed width only do they not — nyxthulhu
– nyxthulhu, Commented Jul 4, 2012 at 21:41
The regex module contains a new regular expression implementation which allows variable-length lookbehind. — BrenBarn
– BrenBarn, Commented Jul 4, 2012 at 21:59
What if the code is part of a URL fragment? /mysite.com/index.php#c99 — Ian Clelland
– Ian Clelland, Commented Jul 4, 2012 at 22:26
Also, something about parsing HTML using regular expressions. — Ian Clelland
– Ian Clelland, Commented Jul 4, 2012 at 22:26

BrenBarn · Accepted Answer · 2012-07-04 22:09:16Z

2

Using the regex module:

>>> rx = regex.compile(r"(?<!#\d{0,3})c99")
>>> rx.findall("#000c99")
[]
>>> rx.findall("/mysite.com/c99.php")
[u'c99']
>>> rx.findall("%20c99.php")
[u'c99']
>>> rx.findall("c99?hi=moo")
[u'c99']

answered Jul 4, 2012 at 22:09

BrenBarn

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Comments

user1502237 · Accepted Answer · 2012-07-04 21:46:54Z

1

The most straight forward way would be to match lines with "c99" in them then discard any where the c99 is in a color code:

line = fileHandle.readline()
while (line) :
     if (re.search("c99", line)) :
          if (re.search("#.?.?.?c99", line)) :
               pass
          else :
               # line contains c99 not in a color code
     line = fileHandle.readline()

There's probably a way to do it within a single regex, but this was just the first thing that came to mind.

answered Jul 4, 2012 at 21:46

user1502237

1 Comment

nyxthulhu Over a year ago

Thanks, I'm trying to do it regex only so that I can avoid writing a macro language system, it won't be the only pattern its used for but in the same circumstance.

burning_LEGION · Accepted Answer · 2012-07-04 22:20:47Z

0

use this regex (^([^#].*?)?c99.*?$)

edited Jul 4, 2012 at 22:20

answered Jul 4, 2012 at 21:57

burning_LEGION

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3 Comments

BrenBarn Over a year ago

This will fail to match cases where c99 occurs on a line with a hex color code before it (e.g., it will not match "#000FFF followed by c99"). Also it matches the entire line, not the c99 itself.

burning_LEGION Over a year ago

i expect that all text would be splitted into words, or regex would be hard and long

BrenBarn Over a year ago

That's a rather big assumption. Anyway, the regex needn't be too huge even if you want to find c99 in arbitrary text.

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