Silly question, but why [0123456789] instead of [0-9]?

Probably because I didn't know regex that well back when I wrote this.
¯_(ツ)_/¯

Thank you! I'm using this to validate call signs in a program to create accounts on a ham radio club station computer.

Thanks for providing this! I modified the expression to not match calls like: A6AA, A6AAA, AAA6A since these don't follow the U.S. rules. Also calls in the range AM* to AZ* don't get matched either.

Modified RegEx: ^[KNWknw][a-zA-Z]{0,2}[0-9][a-zA-Z]{1,3}$|^[Aa][a-lA-L]{1}[0-9][a-zA-Z]{1,2}$

This matches the whole string too. Been using this to validate U.S. callsigns in the DMR database as well as finding several malformed U.S. calls.

@JoshuaCarroll Just saw your comment. Learning Flutter and making a simple call sign lookup iOS and Android app.

Very cool! Thanks for sharing!

According to the Radio Regulations of the ITU, Section III, Article 19, a regular expression for callsigns is:

([BFGKIMNRW]|[0-9][A-Z]|[A-Z][0-9]|[A-Z][A-Z])[0-9][0-9A-Z]*[A-Z]

This, of course, leaves some call signs that are or have been used such as D4A, JY1, 7Q1, or 3DA0RS.

I have not included the prefixes used when operating abroad such as in CT7/EA5IYL or suffixes such as /P, /M, /MM, /B, or /3.

I prefer my own (no offense, please!), it seems to be ITU compliant from what I can tell, allowing only valid prefixes, etc. I might expect a little pushback for this, but this should catch most current (as of 10/2025) legit blocks. I haven't seen it fail, I've tried it on a few international calls, but 99% of what I deal with is scrapping U.S. calls from Netlogger AIM windows, etc. I've tested it with above-average success in a large ball of Lorem Ipsum. I embedded many callsigns and it didn't get fooled by tangled calls+text, e.g.:

WILL NOT find n5abc in:
montes, nascetur ridiculus mus. Morbi placerat sem arcu, fermentum aliquam ligula imperdiet vitae. Nulla ac quam sed risus dignissim cursus ut non ipsum. Etiam mauris justo, semper vitae lacus id, efficitur fringilla ex. Integer aliquet non justo ut semper. n5abcPraesent ut attis orci. Curabitur justo enim, dictum quis est non, vulputate sollicitudin tellus. Integer eget justo in nisi porta egestas sagittis vel quam. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Curabitur a nulla lobortis, egestas sem sed, suscipit neque.
(Etc.)

But WILL find it n5abc in:
montes, nascetur ridiculus mus. Morbi placerat sem arcu, fermentum aliquam ligula imperdiet vitae. Nulla ac quam sed risus dignissim cursus ut non ipsum. Etiam mauris justo, semper vitae lacus id, efficitur fringilla ex. Integer aliquet non justo ut semper. n5abc Praesent ut attis orci. Curabitur justo enim, dictum quis est non, vulputate sollicitudin tellus. Integer eget justo in nisi porta egestas sagittis vel quam. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Curabitur a nulla lobortis, egestas sem sed, suscipit neque. (Etc.)

\b(?:
  A(?:A-L|M-O|P-S|X|Y|Z)|
  B(?:A-Z)?|
  C(?:A-Z)?|
  D(?:A-Z)?|
  E(?:2|3|4|5|6|7|8|9|A-Z)?|
  F(?:A-Z)?|
  G(?:A-Z)?|
  H(?:2|3|4|6|8|9|A-Z)?|
  I(?:A-Z)?|
  J(?:A-Z)?|
  K|
  L(?:A-Z)?|
  M|
  N|
  O(?:A-Z)?|
  P(?:2|3|4|5|6|7|8|9|A-Z)?|
  R|
  S(?:A-Z)?|
  T(?:2|3|4|5|6|7|8|9|A-Z)?|
  U(?:A-Z)?|
  V(?:A-Z)?|
  W|
  X(?:A-Z)?|
  Y(?:A-Z)?|
  Z(?:2|3|4|5|6|7|8|9|A-Z)?
)\d[A-Z]{1,4}\b

The ITU allocation table is pretty big and includes ranges that I believe are best represented as compact ranges (e.g., AA–AL, JA–JS, VK–VN), plus lots of numeric/alpha mixes. I compressed those ranges above into compact patterns so the final regex remains maintainable. The underlying authoritative source is the ITU table (Appendix 42) — you can fetch the latest on your own if this is stale data when you read it!

But if your engine doesn't support the x/free-spacing mode, try this:

\b(?:A(?:A-L|M-O|P-S|X|Y|Z)|B(?:A-Z)?|C(?:A-Z)?|D(?:A-Z)?|E(?:2|3|4|5|6|7|8|9|A-Z)?|F(?:A-Z)?|G(?:A-Z)?|H(?:2|3|4|6|8|9|A-Z)?|I(?:A-Z)?|J(?:A-Z)?|K|L(?:A-Z)?|M|N|O(?:A-Z)?|P(?:2|3|4|5|6|7|8|9|A-Z)?|R|S(?:A-Z)?|T(?:2|3|4|5|6|7|8|9|A-Z)?|U(?:A-Z)?|V(?:A-Z)?|W|X(?:A-Z)?| Y(?:A-Z)?|Z(?:2|3|4|5|6|7|8|9|A-Z)?)\d[A-Z]{1,4}\b