Question

I am looking for a regex for some filename parsing in order to count the number of instances that a filename prefix occurs. Here are some sample strings

gloves.tga
10jeans.jpg
shirt1.png
shirt2.png
coat_00.png
coat_12.gif
top1_01.png
top2_04.png

The basic pattern is just a string of letters or numbers followed by an extension. The prefix is everything before the extension (excluding the period)

A single piece of clothing may be spread across multiple files, indicated by the clothing name, followed by an underscore, followed by some index numbers and then the extension. The prefix is everything up to but not including the underscore. Everything else can be ignored.

This covers all of the cases I’m working with, but I’m having trouble working with the fact that one case has an underscore while the other case doesn’t.

Can someone help me come up with a regex for this?

EDIT: There seems to be an extra condition: shirt1 and shirt2 should be treated as the same prefix.

So if a string is followed by some numbers, and immediately followed by an extension, then the numbers should be ignored, whereas if the numbers were followed by an underscore, then they would be kept in the prefix.

Answer 1

Won’t this work? (Perl/PCRE syntax)

/^([^._]+)/ 

That will capture the longest prefix of the string that contains no periods or underscores.

EDIT: OK, if shirt is the prefix in shirt1, then you can try something like this:

/^([^._]+)(?<!\d)/

which disallows prefixes that end in a digit. That won’t work in Ruby 1.8, though, since 1.8 doesn’t have lookbehind assertions.

EDIT 2:
The above means that the prefix of top1_01 is top, but we want that one to include the digits before the underscore. So our last attempt is to add an alternative:

/^([^._]+)(?:(?<!\d)|(?=_))/

The prefix has to either not end in a digit or be followed by an underscore.
Demo:

%w<gloves.tga  10jeans.jpg shirt1.png  shirt2.png 
   coat_00.png coat_12.gif top1_01.png top2_04.png>.each do |filename|
  if m = filename.match(/^([^._]+)(?:(?<!\d)|(?=_))/) then
    puts [ filename, m[1] ].join ":\t"
  else
    warn "Uh-oh, couldn't find a prefix in filename '#{filename}'."
  end
end    

Output:

 gloves.tga:    gloves
 10jeans.jpg:   10jeans
 shirt1.png:    shirt
 shirt2.png:    shirt
 coat_00.png:   coat
 coat_12.gif:   coat
 top1_01.png:   top1
 top2_04.png:   top2