I guess you'd have to create a group in the regex you can then refer to with \1:
The parentheses mean that whatever is found inside will be equivalent to \1 (or \2, \3, etc if you have more groups).
The exact syntax may vary between different regex dialects. In vim, for instance, you'd have to escape the parentheses: [/]\s\([a-z]\) Check whatever editor you are using.