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POPython Regular Expression - right-to-left
 

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    <p>I am trying to use regular expressions in python to match the frame number component of an image file in a sequence of images. I want to come up with a solution that covers a number of different naming conventions. If I put it into words I am trying to match the last instance of one or more numbers between two dots (eg .0100.). Below is an example of how my current logic falls down:</p> <pre><code>import os import re def sub_frame_number_for_frame_token(path, token='@'): folder = os.path.dirname(path) name = os.path.basename(path) pattern = r'\.(\d+)\.' matches = list(re.finditer(pattern, name) or []) if not matches: return path # Get last match. match = matches[-1] frame_token = token * len(match.group(1)) start, end = match.span() apetail_name = '%s.%s.%s' % (name[:start], frame_token, name[end:]) return os.path.join(folder, apetail_name) # Success eg1 = 'xx01_010_animation.0100.exr' eg1 = sub_frame_number_for_frame_token(eg1) # result: xx01_010_animation.@@@@.exr # Failure eg2 = 'xx01_010_animation.123.0100.exr' eg2 = sub_frame_number_for_frame_token(eg2) # result: xx01_010_animation.@@@.0100.exr </code></pre> <p>I realise there are other ways in which I can solve this issue (I have already implemented a solution where I am splitting the path at the dot and taking the last item which is a number) but I am taking this opportunity to learn something about regular expressions. It appears the regular expression creates the groups from left-to-right and cannot use characters in the pattern more than once. Firstly is there anyway to search the string from right-to-left? Secondly, why doesn't the pattern find two matches in eg2 (123 and 0100)?</p> <p>Cheers</p>
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    1. USYani
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    1. COIs this better? `pattern = r'\.(\d+\.)+'`
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      1. UShughdbrown
    2. CO@hughdbrown That is not his issue...look up `finditer`. Anyways @Yani you should look up `finditer` too. :) It returns an iterator over all *non-overlapping* matches. His second string is `'xx01_010_animation.123.0100.exr'`. The list called `matches` only returns 1 element because the matches overlap.
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    3. COI'm not sure I'm understanding the code you give, but my first thought for searching a string right-to-left would be to reverse the string (`example[::-1]`). Since your pattern (`\.(\d+)\.`) is symmetrical it seems like it would work. Has that option already been ruled out?
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      1. USAli Alkhatib
 

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