Python - Regex - Matching all beginning sequence excluding another pattern

Question

Goal: return grouping that matches all the beginning sequence but excluding a size sequence.

## List of strings and desired result
strs = [
   '151002 - Some name',       ## ('151002 - ', 'Some name')
   'Another name here',        ## ('', 'Another name here')
   '13-10-07_300x250_NoName',  ## ('13-10-07_', '300x250_NoName')
   '728x90 - nice name'        ## ('', '728x90 - nice name')
]

Attempted Pattern

## This pattern is close
## 
pat = '''
^                       ## From start of string
(                       ## Group 1
   [0-9\- ._/]*         ## Any number or divider
   (?!                  ## Negative Lookahead
      (?:\b|[\- ._/\|]) ## Beginning of word or divider
      \d{1,3}           ## Size start
      (?:x|X)           ## big or small 'x'
      \d{1,3}           ## Size end
   )           
)
(                       ## Group 2
   .*                   ## Everthing else
)
'''

## Matching
[re.compile(pat, re.VERBOSE).match(s).groups() for s in strs]

Attempted Pattern Result

[
   ('151002 - ', 'Some name'),      ## Good
   ('', 'Another name here'),       ## Good
   ('13-10-07_300', 'x250_NoName'), ## Error
   ('728', 'x90 - nice name')       ## Error
]

tblznbits · Accepted Answer · 2015-10-22 22:42:50Z

3

I think this might give you what you want:

[re.match(r"^([^x]+[\-_]\s?)?(.*$)", s).groups() for s in strs]

Explanation of regex: Start at the beginning of the string, look for one or more characters that aren't an x that are followed by a hyphen or underscore and possibly followed by a space. That's group one and there can be zero or one of those. Group two is everything else.

EDIT:

Assuming that your strings can have something other than the letter x amongst the numbers, you can modify the code to this:

[re.match(r"^([^a-zA-Z]+[\-_]\s?)?(.*$)", s).groups() for s in strs]

edited Oct 22, 2015 at 22:42

answered Oct 22, 2015 at 22:36

tblznbits

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2 Comments

propjk007 Over a year ago

thanks for the input! While this regex does not appear to meet what I was looking for (see HERE ) your approach does remind me that there are multiple ways to address the issue. Your approach is more a holistic while mine is trying to be very precise. I can see the benefits of this style. Appreciate your post!

tblznbits Over a year ago

@propjk007 Based on the link you provided in your comment, I am assuming that the extra space after the hyphen resulted in a bad output, yes? If so, we can fix that by moving the space outside of the first captured group, like so: [re.match(r"^([^a-zA-Z]+[\-_])?\s?(.*$)", s).groups() for s in strs], which gives us the desired output. You're right that there are multiple ways to address the same issue, especially when regex is involved. I'm happy you were able to find a solution to your problem.

R Nar · Accepted Answer · 2015-10-22 23:22:35Z

1

i think you misunderstand the use of lookaheads. This pattern should work

((?:(?!\d{1,3}x\d{1,3})[0-9\- ._/])*)(.*)

Regular expression visualization

Debuggex Demo

if you want an explanation, because I know it is a disgusting regex, just ask :)

answered Oct 22, 2015 at 23:22

R Nar

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3 Comments

propjk007 Over a year ago

Wow! @r-nar what an amazing tool! Thank you very much for the share! I still do not get how to use the lookahead your example and tool gets me closer. : ) It seems like every example I came across on the web used the lookahead as a do not include (so in my example if any string had the size--i.e. 300x250--the pattern would fail). So of course following and modifying their logic I put the lookahead in the front of the desired pattern. Do you have any good lookahead references?

R Nar Over a year ago

i dont really have a good reference but if it helps, think of the lookahead/lookbehind statements as a probe. Whenever the regex reaches one, it will keep it's current position while using 'another' marker to go ahead or behind the string and match whatever is inside the lookahead statement.

R Nar Over a year ago

also, i use rexegg.com for any of my regex questions, its a good overview site of regex and tricks and tips on how to use it

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