flex : regular expression for accepting only numbers, letters , white spaces

I want to write a regular expression to check string that contains only letters,digits,spaces,# and the string length should be minimum 10 and maximum 29 and string should not begin with blank.

please h开发者_Go百科elp me

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This is one possible regular expression, but I don't know flex:

/^[a-zA-Z0-9#][a-zA-Z0-9 #]{9,28}$/