Python replace a string using re.sub only if prefix and suffix matches

Sachin Shetty

I am trying to convert German words to English using custom dictionary. In below code,replace should only happen if the suffix or prefix of the matching word falls in characters

[,\/!?()_1234567890-=+."""' "]

For exampple: Mein should be converted at first but not in MeinName as the prefix and suffix are not characters mentioned above. If there were single word like _Mein or Mein. it need be converted.

import re

str = "Mein ,Name, ist John, Wo23 bist+ ,_du? , MeinName "
replacements = { 'Mein':'my', 'ist':'is', 'Wo':'where', 'bist':'are', 'du':'you', 'is':'iis'}
re.sub('({})'.format('|'.join(map(re.escape, replacements.keys()))), lambda m: replacements[m.group()], str)

Expected output :

my ,name,is John,where23 are+,_you? ,MeinName

Wiktor Stribiżew

You can use

import re
s = "Mein ,Name, ist John, Wo23 bist+ ,_du? , MeinName "
replacements = { "Mein": "my", "ist": "is", "Wo":"where", "bist":"are", "du":"you", "is" :"iis"}
rx = r'(?:{})(?=[,/!?()_0-9\-=+."\s\'])'.format('|'.join(map(re.escape, replacements.keys())))
print (rx)
print ( re.sub(rx, lambda m: replacements[m.group()], s) )
# => my ,Name, is John, where23 are+ ,_you? , MeinName

See the Python demo.

The regex will look like

(?:Mein|ist|Wo|bist|du|is)(?=[,/!?()_0-9\-=+."\s\'])

See the regex demo. Details:

(?:Mein|ist|Wo|bist|du|is) - one of the alternative strings
(?=[,/!?()_0-9\-=+."\s\']) - a positive lookahead matching a location that is immediately followed with ,, /, !, ?, ), (, _, a digit, -, =, +, ., ", whitespace and '.