Find and replace, reusing token, in Vim

Question

I'm having trouble crafting the appropriate find/replace regex to use in Vim.

Here's some SQL:

select avg(field_name1), avg(field_name2), avg(field_name3) from a_table

Oops, I forgot to alias the results:

select avg(field_name1) field_name1,
avg(field_name2) field_name2, avg(field_name3) field_name3 from a_table

So, in Vim, I'm sure there's a handy find and replace to match an arbitrary [a-z]\+_[a-z]\+ inside parenthesis, but I can't figure out how to put that token after the entire match

Answer 1

Try the following:

:s/(\(.*\))/(\1) \1/g

Answer 2

Group the text that you want to use in the substitution/replacement with surrounding \(\), and refer to these back-references in the substitution using \1, \2, etc.

Using [:alnum:] to match alphanumeric characters:

:s/(\([[:alnum:]]\+_[[:alnum:]]\+\))/(\1) \1/g

To apply either regex to the entire file, use:

:%s/(\([[:alnum:]]\+_[[:alnum:]]\+\))/(\1) \1/g

See also :help \1 for more details about using back-references.