How do I enforce that certain characters must be present when there is an optional character before them?

Question

I would like to capture a string that meets the criteria:

1. may be empty

2. if it is not empty it must have up to three digits (-> \d{1,3})

3. may be optionally followed by a uppercase letter ([A-Z]?)
4. may be optionally followed by a forward slash (i.e. /) (-> \/?); if it is followed by a forward slash it must have from one to three digits (-> \d{1,3})

Here's a valid input:

• 35
• 35A
• 35A/44

Here's invalid input:

• 34/ (note the lack of a digit after '/')

I've come up with the following ^\d{0,3}[A-Z]{0,1}/?[1,3]?$ that satisfies conditions 1-3. How do I deal with 4 condition? My Regex fails at two occassions:

• fails to match when there is a digit and a forward slash and a digit e.g .77A/7
• matches but it shouldn't when there isa digit and a forward slash, e.g. 77/

Answer 1

You may use

/^(?:\d{1,3}[A-Z]?(?:\/\d{1,3})?)?$/

See the regex demo

Details

• ^ - start of string
• (?:\d{1,3}[A-Z]?(?:\/\d{1,3})?)? - an optional non-capturing group:
  • \d{1,3} - one to three digits
  • [A-Z]? - an optional uppercase ASCII letter
  • (?:\/\d{1,3})? - an optional non-capturing group:
    • \/ - a / char
    • \d{1,3} - 1 to 3 digits
• $ - end of string.

Visual graph (generated here):