Thanks for the insight, everyone. After studying andye's and dmmiller2k's suggestions, I was able to come up with a somewhat functioning framework. It was tye's comment that really brought it all home, though. \w does not include spaces, but it does include numbers. Doh! (Upon reflection, my example input data file is a very poor example, indeed.) The solution lied in constructing a character class of [\s\Wa-zA-Z]+.

For completeness' sake, following is the final regex:

(                 # $1 - first IP
\d{1,3}     # 1-3 digits
(?:              # grouping for rep count
\.                # dot
\d{1,3}     # 1-3 digits
){3}          # three more times
(?:             # optional extra grouping
\/               # a slash
\d+           # 1st port number
)?              # close extra grouping
)                # close group around 1st IP
[                # set up character class
\s              # whitespace
a-zA-Z    # letters
\W           # non-word characters
]+             # end character class
(                # $2 - 2nd IP
\d{1,3}   # 1-3 digits
(?:            # grouping for rep count
\.              # dot
\d{1,3}   # 1-3 digits
){3}         # three more times
(?:            # optional extra grouping
\/              # a slash
\d+           # 2nd port number
)?              # close extra grouping
)                # close group around 2nd IP
/x )           # specify /x
[download]

Thanks again!

Update: Fiddled with the grammer.

If things get any worse, I'll have to ask you to stop helping me.