Ruby : Applying a function to a RegExp to retrieve its parsed form

Im trying to convert a Ruby file to Python using a parser.

For example parsing 'myvar.to_s' in ruby will give 'str(myvar)' in python.

In my Ruby parser I do it like this:

@line.gsub!(/(\S+)\.to_s/,'str(\1)')

using the method:

def to_s    return @lineend

Now I could like to do the same for parsing a ruby line of code that is used to match a regular expression (RegExp):

mystring =~ /myregex_(.*?)/i

This would translate in python like this :

mystring = re.search(r'myregex_(.*?)', mystring, re.IGNORECASE)

I dont know how to do this in Ruby, and Im not sure if it is even possible.