Interpreting the D grammar

[cym13 via Digitalmars-d]

On Sunday, 2 August 2015 at 14:50:35 UTC, Jacob Carlborg wrote:
> I'm trying to read the D grammar [1] to enhance the D TextMate 
> bundle. If we take the add expression as an example. It's 
> defined like this in the grammar:
>
> AddExpression:
>     MulExpression
>     AddExpression + MulExpression
>     AddExpression - MulExpression
>     CatExpression
>
> And like this in the grammar made by Brian [2]:
>
> addExpression:
>       mulExpression
>     | addExpression ('+' | '-' | '~') mulExpression
>     ;
>
> I'm not so familiar with grammars but this looks like it's 
> recursive. Is it possible to translate this piece of grammar to 
> a regular expression? TextMate uses regular expressions and a 
> couple of enhancements/extensions to define a grammar for a 
> language.
>
> [1] http://dlang.org/grammar.html
> [2] https://rawgit.com/Hackerpilot/DGrammar/master/grammar.html

You can't build a regular expression for any grammar. You can for 
some grammars but those are only a simple subset. For example, 
checking parens balance is impossible with common (not recursive) 
regular expressions only, and even with recursion it soon reaches 
its limitations.