Replace First and Last Word of String in the Most Pythonic Way

Question

Hey guys, I was building a programme for my school computer exercise which involved me to replace the first and last word of a string. Now I've done it using my own indigenous ways but I'm rather interested to learn the most efficient way I could do it using Python

a = "this is the demo sentence."

I'd like the result of my python function to be:

b = "This is the demo Sentence."

The tricky part of it is that there might be spaces on the front or the end of the string. I need those to be preserved.

Here's what I mean:

a = " this is a demonstration sentence. "

The result would need to be:

b = " This is a demonstration Sentence. "

I'm also interested in opinions on whether a regex would do this job better than python's inbuilt methods, or vice versa.

Answer 1

import re
a = " this is a demonstration sentence. "
print(re.sub(r'''(?x)      # VERBOSE mode
             (             # 
              ^            # start of string
              \s*          # zero-or-more whitespaces 
              \w           # followed by an alphanumeric character
              )        
             |             # OR
             (
             \w            # an alphanumeric character
             \S*           # zero-or-more non-space characters
             \s*           # zero-or-more whitespaces
             $             # end of string
             )
             ''',
             lambda m: m.group().title(),
             a))

yields

 This is a demonstration Sentence.