[DFDL-WG] Fw: pattern based lengths - suggested revised language

Regards Steve Hanson Architect, Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh@uk.ibm.com tel:+44-1962-815848 ----- Forwarded by Steve Hanson/UK/IBM on 16/08/2011 15:28 ----- From: Mike Beckerle To: Steve Hanson/UK/IBM@IBMGB Date: 27/07/2011 15:00 Subject: Re: pattern based lengths - suggested revised language I support what you call the conservative approach. I.e. require text when patterns are used. On Jul 27, 2011 5:53 AM, "Steve Hanson" wrote:

Hi Mike

I don't think we can reduce the wording that much. The second paragraph is needed because it covers the binary case, where encoding is not actually used.

I think we either need to be conservative and disallow the combination of binary & pattern, or leave the second paragraph as-is and effectively say that if you binary with pattern then that is the behaviour.

If we are to be conservative then:

- For a simple element or simple type, disallow lengthKind="pattern" with binary rep.

- For a complex element with lengthKind = "pattern", all children must have lengthUnits = "characters" (so text only) and the encoding of the children must be the same as the encoding of the parent. (We already have a similar rule for complex elements with specified length and lengthUnits = "characters"). We also allow asserts and discriminators to carry patterns which are applied straight at the current position in the data stream. It would be

difficult to police the conservative rules here. But we need to say what

encoding is used and we currently do not. I would say it must be the encoding of the element or group that carries the assert/discriminator. I said on the call that we had extended DFDL regular expressions so that

raw hex bytes could be specified. However I don't see any evidence of this in the DFDL spec. This facility was something we added to IBM MRM for a retail format called TLOG which consists of delimited packed decimal data with hex indicator bytes, so we needed a way to match the hex indicator bytes as part of the regexp. However, I think this was only necessary because MRM has neither speculation nor discriminators, and in a DFDL version of TLOG I would use a discriminator. So I think my statement was

in error, and I don't believe raw hex in DFDL regexps is needed. Regards

Steve Hanson Architect, Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh@uk.ibm.com tel:+44-1962-815848

From: "Mike Beckerle" To: Steve Hanson/UK/IBM@IBMGB Date: 26/07/2011 17:30 Subject: pattern based lengths - suggested revised language

I suggest this language to tighten up this whole section (replace both paragraphs). Given the concerns of Tim, that we make sure DFDL implementations don’t have to reimplement regexp matching, I think this is sufficient. 1.1.1.1 Based Lengths - Scanability Any element (complex, simple text, simple binary) may have a dfdl:lengthKind 'pattern'. When an element contains binary data, and lengthKind=’pattern’ is used, then it is a schema definition error if the character set encoding is not iso-8859-1.

(Possible generalization 1: allow other character sets, e.g., iso-8859-15 as well. This is ok because 8859-15 still maps all 256 codepoints. But this is a slippery slope. )

(Possible generalization 2: allow any character set, Ascii, ebcdic, utf-16be, etc. Note that using any character encoding other than one which maps a valid character to any 8-bit byte creates ambiguity: e.g, the regexp “.” is one where we normally think it means “any character”. But do we really mean “any byte” ? If the character set encoding doesn’t have a given byte as a codepoint, then this question really matters.)

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Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU