c# - Remove the space between single characters only -

i have following string , i'm trying remove space between only single characters, leaving words spaced.

example string

this s e n t e n c e. 

the result needs be:

this sentence. 

another example of:

words must remain spaced **s n g l e** characters **l k e** joined. 

would become:

words must remain spaced **single** characters **like** joined. 

res = regex.replace(s, @"(?<=\b\w)\s(?=\w\b)", "");