3

I have this regex code in python :

if re.search(r"\{\\fad|fade\(\d{1,4},\d{1,4}\)\}", text):
  print(re.search(r"\{\\fad|fade\((\d{1,4}),(\d{1,4})\)\}", text).groups())

text is {\fad(200,200)}Épisode 101 : {\i1}The Ghost{\i0}\Nv. 1.03 and read from a file (don't know if that helps).

This returns the following:

(None, None)

When I change the regex in the print to r"\{\\fad\((\d{1,4}),(\d{1,4})\)\}", it returns the correct values:

(200, 200)

Can anyone see why the conditional fad|fade matches the regex in the re.search but doesn't return the correct values of the groups in the print?

Thanks.

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  • I would consider revising your title such that it's more searchable, Commented Apr 16, 2009 at 20:46
  • Either I'm missing the point of the question, or everyone answering is. Isn't the real question "Why doesn't the regex match when I add groups?" Commented Apr 16, 2009 at 20:50
  • Title changed. Thanks for the suggestion. Commented Apr 16, 2009 at 21:06

6 Answers 6

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6

Put extra parens around the choice: re.search(r"{(?:\\fad|fade)\((\d{1,4}),(\d{1,4})\)}", text).groups()

Also, escaping {} braces isn't necessary, it just needlessly clutters your regexp.

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2 Comments

It works but I don't understand the "(?:" could you clarify, please?
@dex: It's a non-capturing group, so it won't show up when you print the groups.
4

The bracket is part of the or branch starting with fade, so it's looking for either "{fad" or "fade(...". You need to group the fad|fade part together. Try:

r"\{\\(?:fad|fade)\(\d{1,4},\d{1,4}\)\}"

[Edit] The reason you do get into the if block is because the regex is matching, but only because it detects it starts with "{\fad". However, that part of the match contains no groups. You need to match with the part that defines the groups if you want to capture them.

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3 Comments

That was my first guess too, but then why does the first regex match?
The first doesn't match because | has no idea how far back or forward to look, so it looks back to the beginning, and forward to (.
@David Berger: I actually commented on the question to say almost exactly that (and suggest a non-capturing group), but then I realized that I had no idea why that would still let the first regex match, so I deleted it. Thanks for clearing that up.
2

Try this:

r"\{\\fade?\(\d{1,4},\d{1,4}\)\}"
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3 Comments

Certainly a more elegant expression, but doesn't really answer the question.
It fixes the heart of the problem. I think \d{1,4} should still be grouped for capture.
It's matching "{\fad", so the search works but doesn't capture the numbers
1

I think your conditional is looking for "\fad" or "fade", I think you need to move a \ outside the grouping if you want to look for "\fad" or "\fade".

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2 Comments

And if I want to look for "fad" or "fade"? Is there a way without grouping it like (fad|fade)? I mean, I don't want it to be found when calling .groups(). I just want the others values.
Yes, but I think others have already given the correct examples so I'll quit while I'm up... regex's are a minefield:-)
1

Try this instead:

r"\{\\fade?\((\d{1,4}),(\d{1,4})\)\}"

The e? is an optional e. The way you have it now matches {\fad or fade(0000,0000)}

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0

I don't know the python dialect of regular expressions, but wouldn't you need to 'group' the "fad|fade" somehow to make sure it isn't trying to find "fad OR fade(etc..."?

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