Question

I could use some help writing a regular expression. In my Django application, users can hit the following URL:

http://www.example.com/A1/B2/C3 

I’d like to create a regular expression that allows accepts any of the following as a valid URL:

http://www.example.com/A1   http://www.example.com/A1/B2   http://www.example.com/A1/B2/C3   

I’m guessing I need to use the ‘OR’ conditional, but I’m having trouble getting my regex to validate. Any thoughts?

UPDATE: Here is the regex so far. Note that I have not included the ‘http://www.example.com‘ portion — Django handles that for me. I’m just concerned with validating 1,2, or 3 subdirectories.

^(\w{1,20})|((\w{1,20})/(\w{1,20}))|((\w{1,20})/(\w{1,20})/(\w{1,20}))$ 

Answer 1

Skip the |, use the ? and ()

http://www\.example\.com/A1(/B2(/C3)?)?

And if you replace the A1-C3 with a pattern:

http://www\.example\.com/[^/]*(/[^/]*(/[^/]*)?)?

Explanation:

• it matches every string that starts with http://www.example.com/A1
• it can match an additional /B2 and even an additional /C3, but /C3 is only matched, when there is a /B2
• [^/]* (as many non slashes as possible)
• if you need the A1-C3 in special capture groups, you can use this:

http://www\.example\.com/([^/]*)(/([^/]*)(/([^/]*))?)?

Will give (groupnumber: content):

matches: 0: (http://www.example.com/dir1/dir2/dir3) 1: (dir1) 2: (/dir2/dir3) 3: (dir2) 4: (/dir3) 5: (dir3) 

You can check it out online here or get this tool (yes it’s free, and it’s even written in Lisp…).