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  • Great answer. Can easily be applied also in Java using java.util.regex.Pattern.matches("^[a-z_]([a-z0-9_-]{0,31}|[a-z0-9_-]{0,30}\\$)$", user); Commented May 25, 2018 at 6:31
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    It should be [abcdefghijklmnopqrstuvwxyz] instead of [a-z]. [a-z] in many regexp engines also matches things like é, œ or even sometimes multi-character collating elements like dsz in Hungarian locales. Commented Jul 23, 2018 at 6:54
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    Linux usernames do not accept Unicode (unless they are explicitly configured to break POSIX compliance - 1 2). This check should be done outside of the regex, as it's an input/environment/localization validation, not a string validation. Further, I'd love to hear an example of a regex engine that does this. All ones I know of match on ASCII and one has to explicitly enable Unicode, if it's even supported. Commented Jul 24, 2018 at 17:44
  • Don’t apologize for adding value to a thread, even if it is after years of inactivity. But I’m struggling to see just how much value you have added. It seems to me that that your regex is equivalent to the one in Malte Skoruppa’s answer (i.e., in the man page) except yours incorporates the length requirement. One might argue that your version is more compact and efficient (if only in terms of total space); others might say that your version commingles … (Cont’d) Commented Apr 21, 2021 at 22:12
  • (Cont’d) …  the pattern requirement and the length requirement, which could (should?) more simply be handled separately (i.e., the regex from the man page + a call to strlen()). Note that cuonglm’s answer shows the pattern check and the length check being done in separate routines, which seems more modular — a future programmer can change the #define USER_NAME_MAX_LENGTH and not need to mess with the regex. … … … … … … … … P.S. Did you construct your regex yourself? If yes, good job. If not, you should say where you got it. … (Cont’d) Commented Apr 21, 2021 at 22:12