Re: Regular Expressions

Hi Dennis,

Thanks for your comments.

I had three objectives for producing this implementation.

The first objective was to use a single call instead of REGCOMP, REGEXEC and REGFREE.. This is closer to the "standard" implementation found in other languages and scripts. It also makes regular expressions more accessible to those who are unfamiliar with the IBM api's.

My second objective was to remove the confusion / ambiguity caused by differing character sets / ccsid's. While I cannot guarantee that my implementation will work successfully across all the character sets / ccsid's, it is more robust than the IBM-supplied api's.

My third objective was to widen the range of regular expressions available here. Allied to this was the need to improve the documentation for RPG developers.

Two of the examples illustrate a return position rather than 1 (alpha, h and alp25a, \d, both of which return position 4).

Messages which can be of help in determining whether a regular expression is valid or not can be obtained from the return code. A negative return code (range -1 to -18) will indicate the message based on the IBM documentation. These return codes are in the readme, which can be viewed on line.

I agree with you in that "the hard part is in coming up with the right expression". This is the case in all implementations of regular expressions. Different implementations use different (albeit very similar) syntaxes and there is a set of standard escape characters and punctuation characters.

I had considered an implementation of REGEXEC on its own, but this cannot be done without a related implementation of REGCOMP. If you think this to be useful, I will look at it again.


Regards,
John McKay mba

----- Original Message ----- From: "Dennis Lovelady" <iseries@xxxxxxxxxxxx>
To: "'RPG programming on the IBM i / System i'" <rpg400-l@xxxxxxxxxxxx>
Sent: Friday, August 13, 2010 10:03 AM
Subject: RE: Regular Expressions

Thanks, John. I've seen implementations like this before (but not public
domain). I cannot download and restore a save file to the systems I access,
so there is conjecture here. That conjecture can be solved by having a FAQ
for the product. I presume that you're doing REGCOMP, REGEXP, REGFREE with
each operation. Have you measured the performance of this against the
alternative (REGCOMP at the beginning of a process, REGEXP for each of x
million rows/columns/whatevers, REGFREE at the end?

My studies on this (which I unfortunately cannot share) suggest that one
would be much better off using REGCOMP and REGFREE themselves, rather than
taking such a generic approach. After all, the hard part isn't in calling
those expressions or evaluating their results; the hard part is in coming up
with the right expression, and that remains a hurdle with your approach.

In other words, what is the value-add that may be traded for performance?

Your examples, when they find a match, find them in position 1, and return a
1. What happens if a match is found in position n?

When one makes a mistake with a regular expression, what help (optional or
otherwise) does the function return?

Dennis Lovelady
http://www.linkedin.com/in/dennislovelady
--
"Better to remain silent and be thought a fool than to speak out and remove
all doubt."
-- Abraham Lincoln

Single function regular expressions - regexp - now available for free
at www.rpglanguage.com/regexp

Regards,
John McKay mba
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