Question

I’m playing with vim-ruby indent, and there are some pretty complex regexes there:

" Regex used for words that, at the start of a line, add a level of indent.
let s:ruby_indent_keywords = '^\s*\zs\<\%(module\|class\|def\|if\|for' .   
      \ '\|while\|until\|else\|elsif\|case\|when\|unless\|begin\|ensure' . 
      \ '\|rescue\):\@!\>' .                                               
      \ '\|\%([=,*/%+-]\|<<\|>>\|:\s\)\s*\zs' .                            
      \    '\<\%(if\|for\|while\|until\|case\|unless\|begin\):\@!\>'     

With the help of vim documentation I deciphered it to mean:

start-of-line <any number of spaces> <start matching> <beginning of a word> /atom
<one of provided keywords> <colon character> <nothing> <end of word> ...

I have some doubts:

1. Is it really matching ‘:’? Doesn’t seem to work like that, but I don’t see anything about colon being some special character in regexes.
2. why is there \zs (start of the match) and no \ze (end of the match)?
3. what does \%() do? Is it just some form of grouping?

Answer 1

1. :\@! says to match only if there is not a colon, if I read it correctly. I am not familiar with the ruby syntax that this is matching against so this may not be quite correct. See :help /\@! and the surrounding topics for more info on lookarounds.

2. You can have a \zs with no \ze, it just means that the end of the match is at the end of the regex. The opposite is also true.

3. \%(\) just creates a grouping just as \(\) would except that the group is not available as a backreference (like would be used in a :substitute command).