Ruby gsub regex to add the first matched character back into the replaced string

Question

I have a small regex snippet in ruby below which is replacing ": [\w]" with ': ~'

>> "name: Name, phone_number: Phone Number, inactive: Inactive ".gsub(/[:]\s[\w]/, ': ~')

=> "name: ~ame, phone_number: ~hone Number, inactive: ~nactive "

How can I modify the gsub expression to add the first character back into the replaced string, i.e:

=> "name: ~Name, phone_number: ~Phone Number, inactive: ~Inactive "

Thanks

Answer 1

First, you don't need [] around a single character/special character group, as it makes only sense you want to group multiple characters together. Your regex is equivalent to /:\s\w/.

To solve your problem, you can either use a capture group and reinsert the captured letter:

s.gsub(/:\s(\w)/, ': ~\1')
# => "name: ~Name, phone_number: ~Phone Number, inactive: ~Inactive "

Or use a lookahead to not replace the letter in the first place:

s.gsub(/:\s(?=\w)/, ': ~')
# => "name: ~Name, phone_number: ~Phone Number, inactive: ~Inactive "

Maybe you'd rather want to use /:\s+(?=\w)/, which would allow more than one space before the next character.