regex that requires that two matches have the same length

Question

I'd like to find a regex that matches Setex-style headings like:

My Heading
==========

The headings text and the underline need to be of the same length. However, I don't know how to specify a regex that matches the above, but not

No Heading
====

Is there a way to require two groups of the match (first line and second line) to have the same number of characters?

Answer 1

While you could use a regular expression to identify such lines with a specific number of characters, e.g. \v^%(.){6}\n%(\=){6}, there's no way to do so for an arbitrary number of characters with Vim regexes.¹

Hopefully you'll be able to use a Vimscript solution. For example this one liner will save the line numbers of all line pairs that match your criteria:

g/^[=]\+$/ if strlen(getline(line(".") - 1)) == strlen(getline(".")) | call add(b:matches, line(".") - 1) | endif 

It requires existence of a list b:matches.

This one is a little easier to read and initializes the list and prints its contents...

let b:matchlines = []
g/^[=]\+$/ if strlen(getline(line(".") - 1)) == strlen(getline("."))
    call add(b:matches, line(".") - 1)
endif 
echo b:matchlines

Should be easy to tweak this as needed (e.g. if you wanted to save the actual matched text instead of line numbers). If you need some help with such a change let me know.

¹ _{For those hungry for some red meat here's @perelo's comment on why this is so... Actually this is a well-known limitation of theoretical regular expressions. Although pattern matching engines allow more expressivity than th. R.E. (e.g. backreferences with (...)), they cannot match strings as a^nb^n for any n. In your case, 'a' is like . and 'b' is =. Intuitivelly, R.E. are like DFA, so they cannot count dynamically. Patterns such as {n} are obtained by adding O(n) states to the corresponding DFA. This is why we cannot syntactically check a program with a single regex : matching parentheses are like (^n )^n.}