inconsistent matches when capturing group is present

[BurntSushi]

This program

extern crate regex; // 1.0.5
use regex::Regex;

fn main() {
    let text = "foo\nbar\nbaz\n";
    
    let re = Regex::new(r#"(?m)^[ \n]*[a-z]+[ \n]*$"#).unwrap();
    for c in re.captures_iter(text) {
        println!("{:?}", (c.get(0).unwrap().start(), c.get(0).unwrap().end()));
    }
    
    println!("-----------------");
    
    let re = Regex::new(r#"(?m)(^)[ \n]*[a-z]+[ \n]*$"#).unwrap();
    for c in re.captures_iter(text) {
        println!("{:?}", (c.get(0).unwrap().start(), c.get(0).unwrap().end()));
    }
}

outputs

(0, 3)
(3, 7)
(7, 12)
-----------------
(0, 3)
(4, 7)
(8, 12)

but the outputs of each loop should be identical. This is almost certainly a case where the DFA gets it right (the first case) but the Pike VM/backtracker gets it wrong (the second case). My bet is that the compiler is producing bad byte code for the capture group. Taking a look at the byte code:

[andrew@Cheetah regex-debug]$ regex-debug compile '(?m)^[ \n]*[a-z]+[ \n]*$'
0000 Save(0) (start)
0001 StartLine
0002 Split(3, 4)
0003 '\n'-'\n', ' '-' ' (goto: 2)
0004 'a'-'z'
0005 Split(4, 6)
0006 Split(7, 8)
0007 '\n'-'\n', ' '-' ' (goto: 6)
0008 EndLine
0009 Save(1)
0010 Match(0)


[andrew@Cheetah regex-debug]$ regex-debug compile '(?m)(^)[ \n]*[a-z]+[ \n]*$'
0000 Save(0) (start)
0001 Save(2)
0002 StartLine
0003 Save(3)
0004 Split(5, 6)
0005 '\n'-'\n', ' '-' ' (goto: 4)
0006 'a'-'z'
0007 Split(6, 8)
0008 Split(9, 10)
0009 '\n'-'\n', ' '-' ' (goto: 8)
0010 EndLine
0011 Save(1)
0012 Match(0)

If there's an error here, I don't think I see it.

cc @retep998

[davisjam]

Here is a somewhat more minimal example I have independently identified.

Input	/(a)\d*\.?\d+\b/	/a\d*\.?\d+\b/
a0.0c	Matches substring a0.0	Matches substring a0

The presence of the capture group seems to affect the definition of word boundary somehow?

[qbz]

Hello, @BurntSushi, can I ask if there's any progress on this? Thank you! <3

[BurntSushi]

This will be fixed once #656 lands (along with many other bugs).

I have no specific timeline.