Need help with basic functional programming

[Eric via Digitalmars-d-learn]

I have been writing several lexers and parsers. The grammars I 
need to
parse are really complex, and consequently I didn't feel 
confident about
the code quality, especially in the lexers.  So I decided to jump 
on the functional progamming bandwagon to see if that would help. 
 It definitely
does help, there are fewer lines of code, and I feel better about 
the code
quality.  I started at the high level, and had the input buffer 
return a
range of characters, and the lexer return a range of tokens.  But 
when I got
down to the lower levels of building up tokens, I ran into a 
problem:

First I started with this which worked:

private void getNumber(MCInputStreamRange buf)
{
    while (!buf.empty())
    {
        p++;
        buf.popFront();
        if (buf.front() <= '0' || buf.front() >= '9') break;
        *p = buf.front();
    }
    curTok.kind = Token_t.NUMBER;
    curTok.image = cast(string) cbuffer[0 .. (p - 
cbuffer.ptr)].dup;
}

I thought I could improve this like so:

private void getNumber(MCInputStreamRange buf)
{
    auto s = buf.until("a <= '0' || a >= '9'");
    curTok.kind = Token_t.NUMBER;
    curTok.image = to!string(s);
}

The problem is that "until" seems to not stop at the end of the 
number,
and instead continues until the end of the buffer.  Am I doing 
something
wrong here?  Also, what is the fastest way to convert a range to 
a string?

Thanks,

Eric