repetition in regular expression in python

Question

I've got a file with lines for example:

aaa$bb$ccc$ddd$eee
fff$ggg$hh$iii$jj

I need to take what is inside $$ so expected result is:

 $bb$
 $ddd$
 $ggg$
 $iii$

My result:

$bb$
$ggg$

My solution:

m = re.search(r'$(.*?)$', line)
    if m is not None:
        print m.group(0)

Any ideas how to improve my regexp? I was trying with * and + sign, but I'm not sure how to finally create it. I was searching for similar post, but couldnt find it :(

Answer 1

You can use re.findall with r'\$[^$]+\$' regex:

import re
line = """aaa$bb$ccc$ddd$eee
fff$ggg$hh$iii$jj"""
m = re.findall(r'\$[^$]+\$', line)
print(m)
# => ['$bb$', '$ddd$', '$ggg$', '$iii$']

See Python demo

Note that you need to escape $s and remove the capturing group for the re.findall to return the $...$ substrings, not just what is inside $s.

Pattern details:

• \$ - a dollar symbol (literal)
• [^$]+ - 1 or more symbols other than $
• \$ - a literal dollar symbol.

NOTE: The [^$] is a negated character class that matches any char but the one(s) defined in the class. Using a negated character class here speeds up matching since .*? lazy dot pattern expands at each position in the string between two $s, thus taking many more steps to complete and return a match.

And a variation of the pattern to get only the texts inside $...$s:

re.findall(r'\$([^$]+)\$', line) 
               ^     ^

See another Python demo. Note the (...) capturing group added so that re.findall could only return what is captured, and not what is matched.

Answer 2

re.search finds only the first match. Perhaps you'd want re.findall, which returns list of strings, or re.finditer that returns iterator of match objects. Additionally, you must escape $ to \$, as unescaped $ means "end of line".

---

Example:

>>> re.findall(r'\$.*?\$', 'aaa$bb$ccc$ddd$eee')
['$bb$', '$ddd$']
>>> re.findall(r'\$(.*?)\$', 'aaa$bb$ccc$ddd$eee')
['bb', 'ddd']

---

One more improvement would be to use [^$]* instead of .*?; the former means "zero or more any characters besides $; this can potentially avoid more pathological backtracking behaviour.