Question

I’m having a problem in replacing chars in string using python’s 3 regex. I am able to find the pattern occurrences, but I want to replace a char that appears as first in the pattern. Unfortunately I’m replacing the whole pattern. On the other hand – I might be wrong in using regex for this task at all. Here is what I have:

>>> my_table1='\nParametr JednostkaNormaGodzinaŚrednia(1)123456789101112131415161718192021222324 \nDwutlenek siarki (SO2) µg/m3 350 56 53 50 51 51 44 41 36 39 42 34 30 34 33 26 25 24 23 24 25 21 21 22 24 35 \nTlenek azotu (NO) µg/m3 30 30 27 29 44 98 192

What I want to do is to insert ‘,’ or ‘;’ between the numbers. I can not simply replace all spaces with comma as I do not want to split this part: \nDwutlenek siarki (SO2) µg/m3. So I figured to find occurrences of space and digits using regex (r'\s\d+'). This finds all instances correctly. Now I wanted to use the sub function to replace the \s with ‘,’ but I do not know how to isolate just \s from the pattern. Any ideas?

Answer 1

Use lookbehind/lookahead, like this:

p = re.compile(r'(?<=\d)\s(?=\d)')
p.sub(';', my_table1)

Positive lookbehind (?<=\d) matches anything after a digit (\d) without matching the digit itself; \s matches a single whitespace character; and positive lookahead (?=\d) matches anything that is followed by a digit. So this replaces any single whitespace between two digits with a ;. Note that the lookbehind/ahead needs to be fixed length (so you can’t use things like (?<=\d+)).

In your case it should be enough with just r'\s(?=\d)' though, may not need the lookbehind.