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/(.) (a?) (a*) (ab+c) (a{2,6})/

/a (cat|dog)/

[abc]+ [a-zA-Z0-9]+ [^a-zA-Z0-9]+

\d+ \w+ \s+ \D+ \W+ \S+

. ^a a$

贪婪匹配｜懒惰匹配

 <.+> <.+?>

  #[a-fA-F0-9]{6}\b

((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)