From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=unavailable autolearn_force=no version=3.4.4 Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail From: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: Writing a scanner and parser in Ada Date: Tue, 26 Dec 2017 23:20:03 +0100 Organization: Aioe.org NNTP Server Message-ID: References: <2d87298a-4d1f-446a-9d46-d4f03879246b@googlegroups.com> <285de3c1-6c16-47b3-83b7-ec3419c8d324@googlegroups.com> <2c3f6378-1ec4-4128-ad61-a3bcdde49614@googlegroups.com> NNTP-Posting-Host: TliDXSPe+gBSGCqP3SEJ2Q.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 X-Notice: Filtered by postfilter v. 0.8.2 Content-Language: en-US Xref: reader02.eternal-september.org comp.lang.ada:49649 Date: 2017-12-26T23:20:03+01:00 List-Id: On 2017-12-26 19:06, Shark8 wrote: > Let's consider a simple example. We want to write a pattern to test > for a list. We'll define a list as being one or more numbers > separated by comma, and enclosed by parentheses. Use CODE.SNO to try > this definition: > ? ITEM = SPAN('0123456789') > ? LIST = POS(0) '(' ITEM ARBNO(',' ITEM) ')' RPOS(0) > ? '(12,345,6)' LIST > Success > ? '(12,,34)' LIST > Failure > > ARBNO is retried and extended until its subsequent, ')', finally > matches. POS(0) and RPOS(0) force the pattern to be applied to the > entire subject string. I forgot to show the pattern in my syntax: '(' digit $digit $(',' digit $digit) ')' END digit matches '0'..'9'. '$' is an eager repeater similar to 'ARBNO', so $digit is a sequence of digits, maybe empty. digit $digit is a sequence of at least one digit. END matches line end. Matching is always anchored, using SNOBOL terms. BTW, one could consider adding a 'FENCE' or two to prevent backtracking because, and this is the reason why the pattern is that simple, the matching could never give back anything it already matched. It is a kind of problem if you have patterns successfully matching empty strings that are put under a repeater. You may run into an infinite loop with backtracking. As I said it is a toy example which is far easier and more importantly *safer* to write in Ada. And the result will be more readable and far more maintainable in the future. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de