Python: Regex re.sub

re.sub(regex, repl, text)

Substitute regex in text by the replacement repl. If the pattern isn't found, text is returned unchanged.

Returns a new string.

repl can also be a function for more complicated replacement. The function must take a MatchObject as argument. For each occurrence of match, the function is called and its return value used as the replacement string.

re.sub(regex, repl, text, count)

count is the maximum number of pattern occurrences to be replaced.

# example of using re.sub( )

import re

# add alt to image tag
t1 = '<img src="cat.jpg">'
t2 = re.sub(r'src="([a-z]+)\.jpg">', r'src="\1.jpg" alt="\1">', t1)

print(t1) # <img src="cat.jpg">
print(t2) # <img src="cat.jpg" alt="cat">

Function as Replacement

# example of using re.sub(pattern, rep, str ) where rep is a function

import re

def ff(xx):
    if xx.group(0) == "ea":
        return "æ"
    elif xx.group(0) == "oo":
        return "u"
    else:
        return xx.group(0)

print(re.sub(r"[aeiou]+", ff, "encyclopeadia")) # encyclopædia
print(re.sub(r"[aeiou]+", ff, "book")) # buk
print(re.sub(r"[aeiou]+", ff, "geek")) # geek

regex may be a string or an regex object. If you need to specify regular expression flags, you can use a regex object. Alternatively, you can embed a flag in your regex pattern by (?iLmsux) in the beginning of your pattern. e.g. re.sub("(?i)b+", "x", "bbbb BBBB") returns 'x x'. (See: regex pattern syntax for detail.)