HEADER_REGEX
HEADER_REGEX = "(^([^()<>@,;:\\\"/[\\]?={}\x01- ]++):[ \t]*+((?:[ \t]*+[!-~\x80-\xff]++)*+)[ \t]*+\r?\n)m"
Header related regular expressions (copied from amphp/http package) (Note: once we require PHP 7.x we could just depend on the upstream package)
Note: header delimiter (\r\n) is modified to \r?\n to accept line feed only delimiters for BC reasons.