Regex to replace every odd and even occurence in string

Regexes might not be the way to go

iex(1)> str = "_hello_ kevin, please go to chair number _132_ and grab the plate number _4_"
iex(2)> test = Regex.replace(~r/\_(.*?)\_/, str, "<em>\\g{1}</em>") 
"<em>hello</em> kevin, please go to chair number <em>132</em> and grab the plate number <em>4</em>"

This would work if you want to do it with a regex explicitly and you’re sure a “_” will always be used to only start and end a delimitation for substitution.

Many thanks for the answer,

This is the solution, but i have no idea how the "<em>\\g{1}</em>" works.

update:
now I know how it works.

I know I can parse the character one by one.

I just want to know if regex could handle this, and make the code simpler.