# Parser as an alternative to RegExen **URL:** \ **Category:** Ruby\ **Created:** [February 22, 2007, 3:15am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415 "2007-02-22T03:15:22Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 3:15am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/1 "2007-02-22T03:15:22Z")
I’m parsing a large file, currently using compound regexen: PREAMBLE = ‘AA’ USERID = ‘\d{8}’ USER\_HELLO = “#{PREMABLE}(#{USERID})” Is there a simple way to do this using a parser such as ANTLR? I’ve never used one before, so if it requires a learning curve, I’ll stick to my regexen. But if there is a cleaner way to do this, I’d certainly like to. ---
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 4:26am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/2 "2007-02-22T04:26:26Z")
On Feb 21, 2007, at 8:15 PM, S. Robert J. wrote: > I’m parsing a large file, currently using compound regexen: > > PREAMBLE = ‘AA’ > USERID = ‘\d{8}’ > USER\_HELLO = “#{PREMABLE}(#{USERID})” > > Is there a simple way to do this using a parser such as ANTLR? I’ve > never used one before, so if it requires a learning curve, I’ll stick > to my regexen. I really don’t think there’s any value in going all the way to a parser generator here. This job looks to be squarely in the Regexp domain, so there’s no reason to feel bad about using them. James Edward G. II ---
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 4:50am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/3 "2007-02-22T04:50:29Z")
On Thu, Feb 22, 2007 at 12:26:12PM +0900, James Edward G. II wrote: > > to my regexen. > > I really don’t think there’s any value in going all the way to a > parser generator here. This job looks to be squarely in the Regexp > domain, so there’s no reason to feel bad about using them. Agreed. OTOH, Parsers are sure fun to write! (esp. rec descent ones for simple grammars). If you do decide to go with a parser generator, check out Dhaka, [http://dhaka.rubyforge.org/](http://dhaka.rubyforge.org/) ---
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 8:45am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/4 "2007-02-22T08:45:44Z")
On 22.02.2007 04:26, James Edward G. II wrote: > > to my regexen. > > I really don’t think there’s any value in going all the way to a parser > generator here. This job looks to be squarely in the Regexp domain, so > there’s no reason to feel bad about using them. Agree. Also, in Ruby Regexp objects can nicely be used to build larger expressions because Regexp#to\_s is nicely implemented to retain all the settings: irb(main):001:0\> PREAMBLE = /AA/ =\> /AA/ irb(main):002:0\> USERID = /\d{8}/ =\> /\d{8}/ irb(main):003:0\> USER\_HELLO = /#{PREAMBLE}(#{USERID})/ =\> /(?-mix:AA)((?-mix:\d{8}))/ That way you can make sure that all sub expressions are valid and you can nicely mix options - if you need to (for example, preamble case insensitive). Kind regards robert ---
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 9:13am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/5 "2007-02-22T09:13:51Z")
S. Robert J. wrote: > But if there is a cleaner way to do this, I’d certainly like to. As other people has mentioned, there is no biggie using Regexps for this. BUT, another approach which I deem really nice is to use Ragel. Ragel is a generator for Finite State Machines which recently got a backend for Ruby (it’s only in version control yet). The regexps would look almost the same, but the speed would be increase greatly. – Ola B. ([http://ola-bini.blogspot.com](http://ola-bini.blogspot.com)) JvYAML, RbYAML, JRuby and Jatha contributor System Developer, Karolinska Institutet ([http://www.ki.se](http://www.ki.se)) OLogix Consulting ([http://www.ologix.com](http://www.ologix.com)) “Yields falsehood when quined” yields falsehood when quined. ---
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 11:34am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/6 "2007-02-22T11:34:11Z")
On Thu, 22 Feb 2007 03:15:09 +0100, S. Robert J. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > But if there is a cleaner way to do this, I’d certainly like to. One instance where I’d be thinking of picking up parser-fu would be if the data contains recursively nested structures of some sort. Either the regexes, or the ancillary code juggling them gets hairy anyway, losing you the simplicity, and you still have to work your way through the nesting levels manually, which an AST parser would do for you. David V. ---
**Author:** ![robertjames](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e9c0ed/32.png) [@robertjames](https://www.ruby-forum.com/u/robertjames)\ **Post date:** [February 22, 2007, 5:33am UTC](https://www.ruby-forum.com/t/parser-as-an-alternative-to-regexen/86415/7 "2007-02-22T05:33:32Z")
If you’re just looking to get the job done, you should stick with regexes. Your example doesn’t look like it has the kind of expression constructs that would justify applying a full-fledged parser. On the other hand, if you have some time to kill or think your language could get a little more elaborate, then check out Dhaka by all means. 😃 It is still somewhat cumbersome since tokenizers have to be hand-written, but this is about to change. Mushfeq.