regex - blank in regular expression in flex -

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i have following regular expression in flex file. (.l file) 

  doctype [[:blank:] \r\n]*<!doctype[^\[]*\[[[:blank:] \r\n]*

from flex tutorial, in pattern chapiter, saw flex defines ‘[:blank:]’ blank or tab.
in expression above, why need blank after [:blank:]?

the [:blank:] posix character class matches space or tab (even in flex patterns manual).

thus, [[:blank:] \r\n] character class equals [[:blank:]\r\n].

same [rr] = [r].


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