Pattern[/(?x) ^ (? \( (?: [&|] \g+ | ! \g | (?: (? (?: [[:alpha:]] [[:alnum:]_-]* | [[:digit:]]+ (?: \. [[:digit:]]+ )* ) (?: ; [[:alpha:]] [[:alnum:]-]* )* ) [~<>]? = (? (?: [^\\()*] | \\ [[:xdigit:]]{2} )* ) | \g = \* | \g = \g? \* (?: \g \* )* \g? | (?: \g (?: :dn )? (?: : (? [[:digit:]]+ (?: \. [[:digit:]]+ )* | [[:alpha:]] [[:alnum:]]* Match ) )? := \g | (?: : (?: dn | DN ) )? : \g := \g ) ) ) \) ) $/]