Regex for TCP Flags-CodePudding

I'm running into a problem with this regex for TCP Flag validation.

Here is my current version (I've been though many, all are wrong in some way):

^([AFPRSU](?![0-9]))(?!\1)|((?<![A-z])[1-9] [0-9]*)$

So, the value can either be any non-repeating combination of TCP Flags (A,F,P,R,S,U) or the integer equivalent.

Valid:

A,
AFP,
FPURA,
18,
1,
40

Invalid:

empty/null/whitespace,
AA,
aFP,
a100,
100a,
XFP

The problem with my current regex is the lookaheads/behinds only check one character /- so AFPx100 will be a valid value since "F" after "A" is valid. I had similar issues with different iterations, but ultimately it boils down to ignorance. I've been using trial and error on regexr.com but not really getting anywhere, been about 8 hours of effort so far.

I'm unfortunately bound to Regex or I would solve this a different way. At this point I'm desperate for any pointers/advice. Thanks.

CodePudding user response：

Using the anchors to match the whole string, you could match on or more chars out of [AFPRSU] or 1 or more digits starting with a digit 1-9 so that also empty strings do not match:

^(?:(?![AFPRSU]*([AFPRSU])[AFPRSU]*\1)[AFPRSU] |[1-9]\d*)$

Explanation

^ Start of string
(?: Non capture group for the 2 alternatives
- (?![AFPRSU]*([AFPRSU])[AFPRSU]*\1) Negative lookahead, assert not 2 of the same listed characters in the string
- [AFPRSU] Match 1 times any of the listed characters
- | Or
- [1-9]\d* Match a digit 1-9 followed by optional digits
) Close the non capture group
$ End of string

See a regex demo.

const regex = /^(?:(?![AFPRSU]*([AFPRSU])[AFPRSU]*\1)[AFPRSU] |[1-9]\d*)$/;
[
  "A",
  "AFP",
  "FPURA",
  "18",
  "1",
  "40",
  "",
  " ",
  "AA",
  "aFP",
  "a100",
  "100a",
  "XFP"
].forEach(s =>
  console.log(`"${s}" --> ${regex.test(s)}`)
);