find a number in a certain environment in a file and modify it-CodePudding

I have a large file which contains many occurences of certain syntax: (width 0.15).

The number 0.15 can be any floating point number. I wish to find all instances of these numbers and modify them. I know how to modify them using float('0.15') modifier and also I know how to make a string of the result again.

I have trouble finding the numbers. I looked at the re module but don't quite know how to match the numbers themselves and not include the environment with the match.

So I know how to match all the occurrences of the entire environment (width 0.15) using re. I know how to modify the number string. But I don't know how to match for the actual number within the environment. How to do this ?

CodePudding user response：

IIUC, you could use lookarounds:

s = '(width 0.15)'

match = re.search('(?<=\(width )\d (?:\.\d*)?(?=\))', s)

output:

match.group()
# '0.15'

For all occurrences conversion to float:

out = list(map(float, re.findall('(?<=\(width )\d (?:\.\d*)?(?=\))', )s))

output:

[0.15]

regex:

(?<=\(width )  # match should be preceded by "(width "
\d             # one or more digits
(?:\.\d*)?     # optionally, a dot with zero or more digits
(?=\))         # match should be followed by ")"