Reverse marked substrings in a string

Question

I have a string in which every marked substring within < and > has to be reversed (the brackets don't nest). For example,

"hello <wolfrevokcats>, how <t uoy era>oday?"

should become

 "hello stackoverflow, how are you today?"

My current idea is to loop over the string and find pairs of indices where < and > are. Then simply slice the string and put the slices together again with everything that was in between the markers reversed. Is this a correct approach? Is there an obvious/better solution?

Answer 1

It's pretty simple with regular expressions. re.sub takes a function as an argument to which the match object is passed.

>>> import re
>>> s = 'hello <wolfrevokcats>, how <t uoy era>oday?'
>>> re.sub('<(.*?)>', lambda m: m.group(1)[::-1], s)
'hello stackoverflow, how are you today?'

Explanation of the regex:

<(.*?)> will match everything between < and > in matching group 1. To ensure that the regex engine will stop at the first > symbol occurrence, the lazy quantifier *? is used.

The function lambda m: m.group(1)[::-1] that is passed to re.sub takes the match object, extracts group 1, and reverses the string. Finally re.sub inserts this return value.

Answer 2

Or, use re.sub() and a replacing function:

>>> import re 
s = 'hello <wolfrevokcats>, how <t uoy era>oday?'
>>> re.sub(r"<(.*?)>", lambda match: match.group(1)[::-1], s)
'hello stackoverflow, how are you today?'

where .*? would match any characters any number of times in a non-greedy fashion. The parenthesis around it would help us to capture it in a group which we then refer to in the replacing function - match.group(1). [::-1] slice notation reverses a string.