Question

To see what file to invoke the unrar command on, one needs to determine which file is the first in the file set.

Here are some sample file names, of which – naturally – only the first group should be matched:

yes.rar yes.part1.rar yes.part01.rar yes.part001.rar  no.part2.rar no.part02.rar no.part002.rar no.part011.rar 

One (limited) way to do it with PCRE compatible regexps is this:

.*(?:(?<!part\d\d\d|part\d\d|\d)\.rar|\.part0*1\.rar) 

This did not work in Ruby when I tested it at Rejax however.

How would you write one Ruby compatible regular expression to match only the first file in a set of RAR files?

Answer 1

The short answer is that it’s not possible to construct a single regex to satisfy your problem. Ruby 1.8 does not have lookaround assertions (the (?<! stuff in your example regex) which is why your regex doesn’t work. This leaves you with two options.

1) Use more than one regex to do it.

def is_first_rar(filename)     if ((filename =~ /part(\d+)\.rar$/) == nil)         return (filename =~ /\.rar$/) != nil     else         return $1.to_i == 1     end end 

2) Use the regex engine for ruby 1.9, Oniguruma. It supports lookaround assertions, and you can install it as a gem for ruby 1.8. After that, you can do something like this:

def is_first_rar(filename)     reg = Oniguruma::ORegexp.new('.*(?:(?<!part\d\d\d|part\d\d|\d)\.rar|\.part0*1\.rar)')     match = reg.match(filename)     return match != nil end