Regex to match a maximum of 6 digits which may include 1 dash or 1 space anywhere between the digits?

I need help with this:

\d{1,5}(-| )\d{1,5}

This is what I'm working on:

^([C][A|F|G|J|K|L])( |)(\d{1,6}|\d{1,5}(-| )\d{1,5})$

([C][A|F|G|J|K|L]) matches the first 2 characters.

( |) matches an optional space.

\d{1,6} matches 1-6 digits without any dash or space.

\d{1,5}(-| )\d{1,5} is supposed to match a 2-6 digit number which includes either 1 dash or 1 space.

I need help with the last part, but if you know a better way to write the entire Regex, then it should only start with a 'C' and the second character should only be either 'A', 'F', 'G', 'J', 'K' or 'L'. After the first 2 characters, there can optionally be a ' ' (space) character, followed by the 1-6 digits.

Here are examples of what I want to match for the entire Regex:

CA 123456
CF 1
CK 0467 21
CL 4 2
CG 456-789
CJ 1-23

And, if possible (optional), it should also match:

CA546
CJ99-0
CK8 0565

It should NOT match:

CZ 123456
C 987-123
CA 12345-
CK  12
CJ -456
CL-654
CJ 1234567

You can simplify your regex to this,

^C[AFGJKL] ?(?=(?:\d+[ -]\d+$|\d{1,6}$))[\d -]{1,7}$

and achieve your Desired Results

You don't need | when you place characters in a character set [] and single character [C] can simply be written as C

Writing an optional character can be done by any character followed by a ? so optional space can be written as ?

Only tricky part is how do you write \d{1,5}(-| )\d{1,5} in a way so that it either matches only 1 to 6 digits or if there is a single space or hyphen between the digits, then it should match one more additional character to incorporate that space or hyphen so max matched characters become 7 instead of 6, which is where I will explain following regex part,

(?=(?:\d+[ -]\d+$|\d{1,6}$))[\d -]{1,7}

• (?= - positive lookahead so that either of \d+[ -]\d+$ or \d{1,6}$ should match, which essentially says, either match (\d+[ -]\d+$) one or more digits separated with a space or hyphen and again one or more digits else just (\d{1,6}$) match 1 to 6 digits

• [\d -]{1,7} - Finally just capture digits space and hyphen

You could use a single lookahead making sure that there are not 7 consecutive digits.

Then match 1+ digits followed by an optional part matching either a space or - and then again 1+ digits.

Or make it more specific, matching \d{1,5} digits as specified in the question

^C[AFGJKL](?! ?\d{7}) ?\d+(?:[ -]\d+)?$

Explanation

• ^ Start of string
• C Match literally
• [AFGJKL] Match one of the listed characters
• (?! ?\d{7}) Negative lookahead, assert not an optional space followed by 7 consecutive digits
• ? Match an optional space
• \d+(?:[ -]\d+)? Match 1+ digits with an optional part matching either a space or - followed by 1+ digits
• $ End of string

See a regex demo.

I might be wrong, but I think regex doesn't provide a functionality to define a maximum length, and at the same time limit the occurrence of a specific char. Thus, you need to match all possible cases.
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^(C[AFGJKL]) ?(\d{1,6}|\d{1,5}[- ]\d|\d{1,4}[- ]\d{1,2}|\d{1,3}[- ]\d{1,3}|\d{1,2}[- ]\d{1,4}|\d[- ]\d{1,5})$

regex101-link

some further improvements:

• in order to make a matcher optional, you can use x?, which makes the matcher or the group before optional

• when you want to match a char of a set of chars, you can just create a new character class. [asdf] would match any of the letters 'a', 's', 'd', 'f'.