python regex not behaving as i think it should

Question

I am trying to sort through a some files using regex expression.

I have a file which contains the two following lines

NET "MBC_ADR_I1<1>" LOC = "R2";
NET "GP_O<7>" LOC = "R20";

I am using the following expression to get one of the lines only

f2MatchLoc = re.search('(LOC)[ ]+=[ ]+["]?({})'.format(f1LocValue), f2Line, re.IGNORECASE)

where f1LocValue = R2 . However I'm getting a match on both lines.

I've tried to enter the same expression here regex101.com

which shows that my argument should be correctly formatted

Answer 1

f2MatchLoc = re.search(r'(LOC)[ ]+=[ ]+["]?({}\b)'.format(f1LocValue), f2Line, re.IGNORECASE)
                                              ^^

You need to use \\b after R2 so that there are no partial matches. See demo . Also use r or raw mode.

Answer 2

Because you have no conditions how the string should end.

'(LOC)[ ]+=[ ]+["]?({})'
                       ^??

So it matches anything that starts with LOC = "R2 . Following are all valid search results

LOC = "R2 
LOC = "R2asd
LOC = "R2121
LOC = "R2   "

Simply, you can use double quotes or semicolon to identify end of search string. Also you can replace \\s for white-space capturing and you can remove [] around single element lists

r'(LOC)\s+=\s+"?({})"?;'