Rearrange path validation to allow ports

[sttts]

Rebase of #1160

[googlebot]

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[sttts]

@sdboyer anything else you want for this change?

[sttts]

Have to look into the CI errors, something around bzr.

[jmank88]

The AppVeyor bzr failures are pre-existing. I restarted the hung 1.8.x Travis job.

[sdboyer]

i'm fine to eat the cost of unnecessarily rerunning that regex all the time for now. it's a much lower cost than the refactor otherwise required.