Understanding regexes (Was: Re: DMD 0.147 release)

[Georg Wrede]

Walter Bright wrote:
> "Georg Wrede" <georg.wrede at nospam.org> wrote
>> Walter Bright wrote:
>> 
>>> If the compiler is to constant fold regular expressions, then it 
>>> needs to build in to the compiler exactly what would happen if
>>> the regex code was evaluated at runtime.
>> 
>> Yes. IMHO in essence, the binary machine code, which the runtime
>> also would build. What I have a hard time seeing is, how this
>> differs from building a normal function at compile time?
> 
> Consider the strlen() function. Compiling a strlen() function and
> generating machine code for it is a very different thing from the
> compiler knowing what strlen is and replacing:
> 
> strlen("abc")
> 
> with:
> 
> 3

Either I'm getting too old for this business, or you're only giving 
pseudo answers.

(1) If we were to stop the compiler dead in its tracks, and I compiled 
the function "manually" and returned it to the compiler, would we still 
have a problem here?

(2) {-- and this I've so far avoided to bring up, out of courtesy --},
if Don can do it with templates, what's so impossible doing it the 
regular way??

-------------------

Just a cross-check: [I think] we're talking about compiling a single 
regular expression.

My definition: "a compiled regular expression" is any piece of machine 
code that takes *one string* as the argument, and returns (depending on 
which of the 2 kinds it is) either a boolean (as in found or not), or an 
integer denoting position of First Match.

Such a piece of machine code is a function that complies to one of the 
following signatures:

bool foo(char[]);      // match

int bar(char[]);       // search