See /usr/share/common-licenses/GPL-2 

 1. You may make and give away verbatim copies of the source form of the
    software without restriction, provided that you duplicate all of the
    original copyright notices and associated disclaimers.

 2. You may modify your copy of the software in any way, provided that
    you do at least ONE of the following:

      a) place your modifications in the Public Domain or otherwise
         make them Freely Available, such as by posting said
         modifications to Usenet or an equivalent medium, or by
         allowing the author to include your modifications in the
         software.

      b) use the modified software only within your corporation or
         organization.

      c) rename any non-standard executables so the names do not
         conflict with standard executables, which must also be
         provided.

      d) make other distribution arrangements with the author.

 3. You may distribute the software in object code or executable
    form, provided that you do at least ONE of the following:

      a) distribute the executables and library files of the
         software, together with instructions (in the manual page or
         equivalent) on where to get the original distribution.

      b) accompany the distribution with the machine-readable source
         of the software.

      c) give non-standard executables non-standard names, with
         instructions on where to get the original software
         distribution.

      d) make other distribution arrangements with the author.

 4. You may modify and include the part of the software into any
    other software (possibly commercial). But some files in the
    distribution are not written by the author, so that they are not
    under this terms.

    They are gc.c(partly), utils.c(partly), regex.[ch], st.[ch] and
    some files under the ./missing directory. See each file for the
    copying condition.

 5. The scripts and library files supplied as input to or produced as
    output from the software do not automatically fall under the
    copyright of the software, but belong to whomever generated them,
    and may be sold commercially, and may be aggregated with this
    software.

 6. 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.