The flex manual is placed under the same licensing conditions as the rest of flex:
Copyright © 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2012 The Flex Project.
Copyright © 1990, 1997 The Regents of the University of California. All rights reserved.
This code is derived from software contributed to Berkeley by Vern Paxson.
The United States Government has rights in this work pursuant to contract no. DE-AC03-76SF00098 between the United States Department of Energy and the University of California.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED “AS IS” AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.