使用PCRE 2：

(?=^[a-z0-9_.]{8,18}$) (?# Restrict to 8,18 characters by positive lookahead)
^[a-z0-9]+             (?# Must begin with a letter or number )
(?:
  [.][a-z0-9]*_?|      (?# Period then optional underscore)
  _[a-z0-9]*[.]?|      (?# Underscore then optional period)
                       (?# Or Nothing)
)[a-z0-9]+$            (?# Must end with a letter or number)

我有一个想法，如果你添加更多的特殊字符，这个解决方案变得太复杂了。相反，你可以使用更积极的lookaheads。

(?=^.{8,18}$)        (?# Restrict to 8,18 characters by positive lookahead)
(?=^[^.]*[.]?[^.]*$) (?# Restrict to one period by positive lookahead)
(?=^[^_]*[_]?[^_]*$) (?# Restrict to one underscore by positive lookahead)
^[a-z0-9]+           (?# Must begin with a letter or number)
[a-z0-9._]*          (?# All characters allowed in the middle)
[a-z0-9]+$           (?# Must end with a letter or number)