There's now an ANSI standard for REXX. The REXX Standard number is X3.274.
You can get a copy of the Standard from the ANSI at:  American National
Standards Institute
 Attn: Customer Service
 11 West 42nd Street
 New York, NY 10036
 Phone: +1 212 642 4900
 Fax:  +1 212 302 1286
 (see Internet - Web Pages)
The document costs $125 (+S&H).
A cheaper way for retrieving a copy of the standard is:
Download the pre-publication PostScript draft version of the standard from Mike Cowlishaw's REXX home page (see Internet - Web Pages). This version is functional identical to the ANSI version.
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History
"Since 1991, a technical committee has been preparing a document for a proposed American National Standard (ANSI Standard) for REXX. Here is the committee's charter:
The project is for development of an American National Standard. The scope of this standard will be 'The REXX Language' described by Michael F. Cowlishaw in his book, altered as necessary to promote portability, reliability, maintainability and efficient execution of REXX programs on a variety of computing systems."
The X3J18 committee has been operationg under the procedures of the American
National Standards Institute (ANSI); the addresses of the Chairman, Vice-Chairman,
and Secretary are as follows: Chairman:
 B L Marks
 IBM UK Laboratories
 Hursley Park
 Winchester, England SO21 2JN
 rexxarch@winvmd.vnet.ibm.com
Vice-chairman:
 N F N Milsted
 IX Corporation
 575 W. Madison, Suite 3610
 Chicago, IL 60661
 nfnm@wrkgrp.com
Secretary:
 E Spire
 The Workstation Group
 6300 River Road
 Rosemont, IL 60018
 ets@wrkgrp.com
"