Question

Can some please explain the following regexp, which I found in ediff-trees.el as a specification for which files/directories to exclude from its comparison process.

"\\`\\(\\.?#.*\\|.*,v\\|.*~\\|\\.svn\\|CVS\\|_darcs\\)\\'"

Although I am somewhat familiar with regular expressions encountering this elisp string-based variant has thrown me off.

Answer 1

First thing, remember that elisp’s regexes have to be string-escaped, which created a lot of extra backslashes. Removing them, we get

\`\(\.?#.*\|.*,v\|.*~\|\.svn\|CVS\|_darcs\)\'

Then, \( and \) mean grouping, “foo\|bar” means “either foo, or bar”.

So, piece by piece, this regexp matches: either an emacs temporary file (something starting with #, possibly preceded by a period: .?#.), or an RCS file (ending in ,v: .,v), or an emacs backup file (ending in ~: .*~), or an svn directory (.svn), or a cvs directory (CVS), or a darcs directory (_darcs).

Edit to correct: as andre-r correctly points out, the backtick \` and single quote \’ basically mean “beginning and end of the string” (respectively). So this means that the regexp finds strings which match exactly one of the choices I’ve outlined above (i.e., the string starts, then comes one of those choices, then the string ends). I had previously said they meant quoting, I don’t know what I was thinking :). Thanks andre-r!