Q:

Need regular expression to match specific pattern in notepad++

I want search for specific pattern like
'constant_string' : some_string.
'constant_string' : this will be constant part in pattern & some_string it may change. 
I want to find for this pattern 'constant_string' : some_string and replace with 'constant_string' : 'some_string'. (single quote added to some_string)

A:

Try using:
(?<='constant_string' \: )([A-Za-z0-9_]*)

And replace with:
'\1'

Captures the string into a capturing group and uses it to replace the quoted value
Demo : https://regex101.com/r/Hl3Zpe/1
Better Method:
('constant_string' \: )([A-Za-z0-9_]*)

Replace with
'\2'

Using 2 here, since there are two capturing groups and \1 is now constant_string : and the variable string is in \2

