String Operation on captured group in re Python

Question

I have a string:

str1 = "abc = def"

I want to convert it to:

str2 = "abc = #Abc#"

I am trying this:

re.sub("(\w+) = (\w+)",r"\1 = %s" % ("#"+str(r"\1").title()+"#"),str1)

but it returns: (without the string operation done)

"abc = #abc#"

• What is the possible reason .title() is not working.?
• How to use string operation on the captured group in python?

Answer 1

You can see what's going on with the help of a little function:

import re

str1 = "abc = def"

def fun(m):
    print("In fun(): " + m)
    return m

str2 = re.sub(r"(\w+) = (\w+)",
    r"\1 = %s" % ("#" + fun(r"\1") + "#"),
    #                   ^^^^^^^^^^
    str1)

Which yields

In fun(): \1

So what you are basically trying to do is to change \1 (not the substitute!) to an uppercase version which obviously remains \1 literally. The \1 is replaced only later with the captured content than your call to str.title().

Go with a lambda function as proposed by @Rakesh.

Answer 2

Try using lambda.

Ex:

import re
str1 = "abc = def"
print( re.sub("(?P<one>(\w+)) = (\w+)",lambda match: r'{0} = #{1}#'.format(match.group('one'), match.group('one').title()), str1) )

Output:

abc = #Abc#