deciphering vim regex

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).

Answer 2

1. you can check about matching ':' or any other string by copying the regex and using it to perform a search with / on the code you are working. Using :set incsearch may help you to see what is being matched while you type the regex.

2. the \zs and \ze don't affect what is matched, but instead determine which part of matched text is used in functions as :s/substitute(). You can check that by performing searches with / and 'incsearch' option set - you can start a search for a string in the text, which will be highlighted, then adding \zsand \ze will change the highlight on the matched text. There is no need to "close" \zsand \ze, as one can discard only the start or the end of the match.

3. It is a form of grouping that is not saved in temporary variables for use with \1, \2 or submatch(), as stated in :h \%():

   \%(\) A pattern enclosed by escaped parentheses. Just like \(\), but without counting it as a sub-expression. This allows using more groups and it's a little bit faster.