Python Regex – A lookbehind assertion has to be fixed width
Question:
I want to extract a certrain string from a path. The wanted string is always preceded by either _ASW or 10_BSW words. Additionally, the sought string consists of only letters and numbers.
So for example from the following 3 paths I want to extract strings Mod2000, ModA and ModB:
C:MyPath _ASWMod2000
C:MyPath10_BSWModASubDir
C:MyPath10_BSWModB
For that I have written a regex using Positive Lookbehind:
\(?<=(0_ASW|10_BSW)\)([A-Za-z0-9]+)
With this regex the 2nd group matches the sought string correctly and I am able to compile the regex in .NET(C#) without any errors. However, once I try to compile it in Python I get the following Regex Error: A lookbehind assertion has to be fixed width
From my understanding, the two words in the positive lookbehind, i.e. 0_ASW and 10_BSW ought to have the fixed length. The error is not clear to me because both words have a fixed length of 4 and 5 characters, respectively. If I try to make those 2 strings to have equal length, e.g. 3 character strings ASW and BSW, the regex compiles without the above error.
\(?<=(ASW|BSW)\)([A-Za-z0-9]+)
How do I fix this regex so that it compiles in Python as well?
You can find the demos here:
Answers:
\((0_ASW|10_BSW)\)([A-Za-z0-9]+)
You could also use a non-capturing group:
\(?:0_ASW|10_BSW)\(w+)
https://regex101.com/r/hYCRJf/1
If the regex matches, you’ll get the desired string in group(1).
You can use a lookahead like this with an alternation, as for Python it has to be fixed width which they are not in your pattern.
b(?:(?<=\0_ASW\)|(?<=\10_BSW\))[A-Za-z0-9]+
See a regex101 demo.
If you can make use of the PyPi regex module, you match what you want then then you can use K to forget what is matches so far:
\(?:0_ASW|10_BSW)\K[A-Za-z0-9]+
See another regex101 demo.
I want to extract a certrain string from a path. The wanted string is always preceded by either _ASW or 10_BSW words. Additionally, the sought string consists of only letters and numbers.
So for example from the following 3 paths I want to extract strings Mod2000, ModA and ModB:
C:MyPath _ASWMod2000
C:MyPath10_BSWModASubDir
C:MyPath10_BSWModB
For that I have written a regex using Positive Lookbehind:
\(?<=(0_ASW|10_BSW)\)([A-Za-z0-9]+)
With this regex the 2nd group matches the sought string correctly and I am able to compile the regex in .NET(C#) without any errors. However, once I try to compile it in Python I get the following Regex Error: A lookbehind assertion has to be fixed width
From my understanding, the two words in the positive lookbehind, i.e. 0_ASW and 10_BSW ought to have the fixed length. The error is not clear to me because both words have a fixed length of 4 and 5 characters, respectively. If I try to make those 2 strings to have equal length, e.g. 3 character strings ASW and BSW, the regex compiles without the above error.
\(?<=(ASW|BSW)\)([A-Za-z0-9]+)
How do I fix this regex so that it compiles in Python as well?
You can find the demos here:
\((0_ASW|10_BSW)\)([A-Za-z0-9]+)
You could also use a non-capturing group:
\(?:0_ASW|10_BSW)\(w+)
https://regex101.com/r/hYCRJf/1
If the regex matches, you’ll get the desired string in group(1).
You can use a lookahead like this with an alternation, as for Python it has to be fixed width which they are not in your pattern.
b(?:(?<=\0_ASW\)|(?<=\10_BSW\))[A-Za-z0-9]+
See a regex101 demo.
If you can make use of the PyPi regex module, you match what you want then then you can use K to forget what is matches so far:
\(?:0_ASW|10_BSW)\K[A-Za-z0-9]+
See another regex101 demo.