Hello,
I have difficulties with forward back references. For example:
/^abc(d)\1$/
versus
/^abc\1(d)$/
Shouldn't abcdd match with both patterns?
With the first one (actual back reference) it does.
But not with the second one (forward back reference):
re> /^abc(d)\1$/D
------------------------------------------------------------------
0 24 Bra
3 ^
4 abc
10 7 CBra 1
15 d
17 7 Ket
20 \1
23 $
24 24 Ket
27 End
------------------------------------------------------------------
Capturing subpattern count = 1
Max back reference = 1
Options: anchored
No first char
No need char
data> abcdd
0: abcdd
1: d
re> /^abc\1(d)$/D
------------------------------------------------------------------
0 24 Bra
3 ^
4 abc
10 \1
13 7 CBra 1
18 d
20 7 Ket
23 $
24 24 Ket
27 End
------------------------------------------------------------------
Capturing subpattern count = 1
Max back reference = 1
Options: anchored
No first char
No need char
data> abcdd
No match
Ok, man pcrepattern says, named back references can be used in
both ways. But I don't get the results I would have expected
with those ones, either:
re> /^((?P=abc)|X)(?<abc>x|y)+/D
------------------------------------------------------------------
0 35 Bra
3 ^
4 8 CBra 1
9 \2
12 5 Alt
15 X
17 13 Ket
20 7 CBra 2
25 x
27 5 Alt
30 y
32 12 KetRmax
35 35 Ket
38 End
------------------------------------------------------------------
Capturing subpattern count = 2
Max back reference = 2
Named capturing subpatterns:
abc 2
Options: anchored
No first char
No need char
data> Xx
0: Xx
1: X
2: x
data> Xy
0: Xy
1: X
2: y
data> xx
No match
data> xy
No match
Why don't the last ones match, as well?
With the pattern the other way round xx does match:
re> /(?<abc>x|y)((?P=abc)|X)/D
------------------------------------------------------------------
0 34 Bra
3 7 CBra 1
8 x
10 5 Alt
13 y
15 12 Ket
18 8 CBra 2
23 \1
26 5 Alt
29 X
31 13 Ket
34 34 Ket
37 End
------------------------------------------------------------------
Capturing subpattern count = 2
Max back reference = 1
Named capturing subpatterns:
abc 1
No options
No first char
No need char
data> xx
0: xx
1: x
2: x
I refer to pcre-7.4.
Best regards,
Juergen
