replace or operate within regex match

Question

underlying question: Is there a way to operate on text within a regex match? I.e. is there an analog to the 'g' command that operates on match substrings instead of lines

usecases: One example of this would be if you replace a pattern within a pattern without a huge amount of regex gymnastics (which are often possible with much effort). A specific example might be replace spaces with tabs but only in text matching a regex.

field1 field2 field3 field4 field with spaces

Another specific example would be if you wanted to replace - with _ but only in text inside quotes

You should name your file [something something something]
and name their file [a b c d] 

Answer 1

Maybe your solution is to use a sub-replace-expression (:h sub-replace-expression)

For example in your first example let's say that I want to replace spaces with underscores (because it's more visual in this answer) but only in the last field field with spaces:

field1 field2 field3 field4 field with spaces

I could use this command:

:s/field w.*/\=substitute(submatch(0), '_', ' ', 'g')/

 s/                                                     Substitution command
   field w.*                                            Make the substitution only on the part of the string you want to match
            /
             \=                                         Replace with an expression
               substitute(           , ' ', '_', 'g')   Use the substitution() function to replace spaces with underscore
                          submatch(0)                   Give the matched string as parameter of substitute()

This will result in

field1 field2 field3 field4 field_with_spaces

This idea is to use the substitute() function inside the replacement part of the :s command. submatch(0) is a reference to the pattern matched by :s.

In the same idea to replace something something something by something_something_something in your second example you could do this:

:s/\[.*\]/\=substitute(submatch(0), ' ', '_', 'g')/

You might want to read the following help topics:

• :h sub-replace-expression

• :h submatch()

• :h substitute()