regex error

[Dmitry Olshansky]

On Sunday, 23 November 2025 at 12:43:05 UTC, Jabba Laci wrote:
> I'm working on an Advent of Code problem (2015, Day 5, Part 2), 
> and my code doesn't work in D. In Python I get the correct 
> result.
>
> Here is a string:
>
>     const s = "xdwduffwgcptfwad";
>
> The instructions says: "It contains a pair of any two letters 
> that appears at least twice in the string without overlapping, 
> like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it 
> overlaps)."
>
> Here, "fw" appears twice. My D code:
>
>     auto m1 = matchFirst(s, regex(r"(..).*\1"));
>
> returns an empty m1 object. What am I doing wrong? The same 
> regex works in Python.

Author of std.regex here. Indeed it’s unfortunate that the main 
engine doesn’t support all cases of backreferences. It’s been a 
while since I touched std, but the fix should be mostly 
strightforward - use simple backtracking for this cases. 
std.regex has backtracking but I think it’s also augmented with 
certain tricks to avoid exponential behaviour. Those need to be 
reverted, then std.regex could just select simple backtracking 
for patterns that have backreferences.

—
Dmitry Olshansky