How do I insert space before capital letter if and only if previous letter is not capital?

Question

I have the text:

'SMThingAnotherThingBIGCapitalLetters'

and I want the output to be:

'SM Thing Another Thing BIG Capital Letters'

My regex now:

r"(\w)([A-Z])", r"\1 \2"

This works when I don't have 2 capital letters near eachother.

Output for my regex:

'S MThing Another Thing B I G Capital Letters'

So, I need regex to insert a space before a capital letter when next letter is small.

Anyone have an idea?

Answer 1

You should use regular expressions carefully. They can easily transform to gargantuan monsters nobody can understand. You can solve your problem with simple loop instead of regexp:

a = 'SMThingAnotherThingBIGCapitalLetters'
result = a[0]

for i, letter in enumerate(a):
    if letter.isupper() and (result[-1].islower() or a[i+1].islower()):
        result += ' '
    if i: result += letter
result

'SM Thing Another Thing BIG Capital Letters'

Answer 2

You could use alternation with 2 capturing groups and replace with group1 group2 space like r"\\1\\2 "

([A-Z])(?=[A-Z][a-z])|([a-z])(?=[A-Z])

Explanation

• ([AZ]) Capture captital AZ in group 1
• (?=[AZ][az]) Positive lookahead, assert what is on the right is an uppercase and a lowercase az
• | Or
• ([az]) Capture lowercase az in group 2
• (?=[AZ]) Positive lookahead, assert what is on the right is uppercase AZ

Regex demo