Question

So I wanted to limit a textbox which contains an apartment number which is optional.

Here is the regex in question:

([0-9]{1,4}[A-Z]?)|([A-Z])|(^$)

Simple enough eh?

I’m using these tools to test my regex:
Regex Analyzer
Regex Validator

Here are the expected results:

• Valid
  • “1234A”
  • “Z”
  • “(Empty string)”
• Invalid
  • “A1234”
  • “fhfdsahds527523832dvhsfdg”

Obviously if I’m here, the invalid ones are accepted by the regex. The goal of this regex is accept either 1 to 4 numbers with an optional letter, or a single letter or an empty string.

I just can’t seem to figure out what’s not working, I mean it is a simple enough regex we have here. I’m probably missing something as I’m not very good with regexes, but this syntax seems ok to my eyes. Hopefully someone here can point to my error.

Thanks for all help, it is greatly appreciated.

Answer 1

You need to use the ^ and $ anchors for your first two options as well. Also you can include the second option into the first one (which immediately matches the third variant as well):

^[0-9]{0,4}[A-Z]?$

Without the anchors your regular expression matches because it will just pick a single letter from anywhere within your string.