Question

Our web application allows users to specify their own “slugs” which can include relative paths e.g. /somedir/some-file.htm.

In our routing configuration we need to ensure that only valid slugs (with segments) are supported.

The regex I am using is:

(^[a-z0-9])([a-z0-9-/]+)([a-z0-9])$

This means:

• A valid slug will match e.g. some-file.htm
• A valid slug with segments (relative path) will match e.g. somedir/subdir/some-file.htm
• Absolute URLs will NOT match e.g. /somedir/some-file.htm
• Trailing / or – will not match e.g. -slug-

Unfortunately it also means that double slashes will match e.g. somedir//subdir//some-file.htm because my expression is allowing one or more slashes.

How can I change it to allow zero or more slashes between segments.

I thought that:

(^[a-z0-9])(/?[a-z0-9-]+/?)([a-z0-9])$

would work but it does not.

Answer 1

^[a-z0-9]([a-z0-9-]*[a-z0-9])?(/[a-z0-9]([a-z0-9-]*[a-z0-9])?)*$

EDIT: Use this one if you like the first regex:

^(?!-)[a-z0-9-]+(?<!-)(/(?!-)[a-z0-9-]+(?<!-))*$

It looks messy and complicated, but it seems to be correct per your spec.

[a-z0-9]([a-z0-9-]*[a-z0-9])?

Matches a single name. Ignoring /s for the moment.

Then the rest of it is a single slash followed by that same thing again.

As mentioned in Karoly’s answer, this does not include literal periods, for instance “some-file.htm” will not match the regex I wrote.

If this is desired behavior then you’ll actually want:

^[a-z0-9]([a-z0-9-\.]*[a-z0-9])?(/[a-z0-9]([a-z0-9-\.]*[a-z0-9])?)*$

Finally, if you want to allow literal periods in only the last section then you’ll want:

^[a-z0-9]([a-z0-9-]*[a-z0-9])?(/[a-z0-9]([a-z0-9-]*[a-z0-9])?)*(/[a-z0-9]([a-z0-9-\.]*[a-z0-9])?)?$

EDIT:

A thought occurs that this can be simplified a bit using lookaheads and behinds.

^[a-z0-9]([a-z0-9-]*[a-z0-9])?(/[a-z0-9]([a-z0-9-]*[a-z0-9])?)*(/[a-z0-9]([a-z0-9-\.]*[a-z0-9])?)?$

becomes:

^(?!-)[a-z0-9-]+(?<!-)(/(?!-)[a-z0-9-]+(?<!-))*(/(?!-\.)[a-z0-9-\.]+(?<!-\.))?$