Question

I want to build a regex that will match the following strings:

A string with all or some of the 4 characters “A”,”B”,”C” and “D”

the “ABCD” can occur at any position at most once.
For example, the following are expected to match:

A,AB,CABD,DC,BDCA,ABC,...

The following should not match:

ABA,BB,ABCC,DDAA,AACD...

Do you have any suggestion for me?

Answer 1

This one should do it (Python syntax):

if re.match(r"""
        # Match string having max one each of A, B, C or D.
        ^                      # Anchor to start of string.
        (?=[^A]*(?:A[^A]*)?$)  # Assert zero or one A max.
        (?=[^B]*(?:B[^B]*)?$)  # Assert zero or one B max.
        (?=[^C]*(?:C[^C]*)?$)  # Assert zero or one C max.
        (?=[^D]*(?:D[^D]*)?$)  # Assert zero or one D max.
        [ABCD]+                # One or more of [ABDC].
        \Z                     # Anchor to end of string.
        """, text, re.VERBOSE):
    # Successful match at the start of the string
else:
    # Match attempt failed

Here is a JavaScript version:

var re = /^(?=[^A]*(?:A[^A]*)?$)(?=[^B]*(?:B[^B]*)?$)(?=[^C]*(?:C[^C]*)?$)(?=[^D]*(?:D[^D]*)?$)[ABCD]+$/;
if (re.test(text)) {
    // Successful match
} else {
    // Match attempt failed
}