m!            # Use an alternate delimiter.
  /           # Match a '/' character to anchor off of.
  (           # Start a capturing parenth.
    [^/]+     # Match any number of non-'/' characters.
    \.        # Match the '.' character.
    (?:       # Group or constrain without capturing.
        jpe?g # jpg or jpeg.
      | gif   # or gif
      | png   # or png
      | tiff? # or tif or tiff
    )         # End the grouping/constraining parens.
    \b        # Make sure that tiff isn't tiffany, for
              # example.
  )           # End the capture.
!ix           # Case insensitive /i (Ignore the x, it's only
              # necessary in this expanded example.