Question

I have this regular expression to validate the URL: ^[a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,3}(/\S*)?$^. This regular expression works smoothly but I want to add a limit the amount of W’s in the beginning of the URL.

If the user tries to save the URL with under 3 W’s (for example ww), the regular expression will deny the save. The same result will also happens if the user tries to save the URL with more than 3 W’s (for example wwww).

How can I solve this problem?

Thanks in advance.

Answer 1

This sort of filtering is not a good fit for regular expressions, I think.

The problem is that the rules for what should be a “match” are actually pretty complicated. Essentially the rules are this:

Match something if it has :

• start of text
• either:
  • exactly three w characters followed by a dot OR
  • any alphanumeric characters or dots repeated any number of times, followed by a dot
    • unless all of those characters are w characters, but the number of characters is not equal to three (plus the dot)
• two or three more alphanumeric characters
• end of text

The unless all of those characters are w characters… part is the tricky part. Regex isn’t really well suited to this task.

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For “historical” purposes:

Use {n} to repeat part of the expression n times.

^w{3}\.[a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,3}(\/\S*)?$

Use ? to make part of the expression optional.

The parentheses are a grouping operator. The “w times three” and dot are moved inside the group, and the group is made optional with the ? operator.

^Guess what? this doesn't work.$
^I tried to delete the answer but I can't until you unaccept it.$

I also escaped the last forward slash with a backslash in these examples, since regular expressions are often delimited with / characters. You can remove it if you don’t need it.