Good commentary AnomalousMonk :)

This is where something like rxrx shines over use re 'debug'; as it shows you how the matching goes without the interference of optimizations :)

 Restarting regex match
 |
 V
/(?=4)W/


 |
 V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 0]
---
 Starting positive lookahead
 |
 VVV
/(?=4)W/


 |
 V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 1]
---
    Trying a literal '4' character
    |
    V
/(?=4)W/


 |
 V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 2]
---
    Failed
    |
    V
/(?=4)W/


 |
 V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 3]
---
 Back-tracked within regex and rerestarting regex match
 |
 V
/(?=4)W/


  |
  V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 4]
---
 Starting positive lookahead
 |
 VVV
/(?=4)W/


  |
  V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 5]
---
    Trying a literal '4' character
    |
    V
/(?=4)W/


  |
  V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 6]
---
    Matched
    |
    V
/(?=4)W/


   |
   V
'54w'
  ^

[Visual of regex at 'rxrx' line 0]         [step: 7]
---
     Back-tracked within regex and reend of positive lookahead
     |
     V
/(?=4)W/


  |<
  V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 8]
---
      Trying a literal 'W' character
      |
      V
/(?=4)W/


  |
  V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 9]
---
      Failed
      |
      V
/(?=4)W/


  |
  V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 10]
---
 Back-tracked within regex and rerestarting regex match
 |
 V
/(?=4)W/


   |
   V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 11]
---
 Starting positive lookahead
 |
 VVV
/(?=4)W/


   |
   V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 12]
---
    Trying a literal '4' character
    |
    V
/(?=4)W/


   |
   V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 13]
---
    Failed
    |
    V
/(?=4)W/


   |
   V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 14]
---
 Back-tracked within regex and rerestarting regex match
 |
 V
/(?=4)W/


    |
    V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 15]
---
 Starting positive lookahead
 |
 VVV
/(?=4)W/


    |
    V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 16]
---
    Trying a literal '4' character
    |
    V
/(?=4)W/


    |
    V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 17]
---
       Regex failed to match after 18 steps
       |
       V
/(?=4)W/


    |
    V
'54w'


[Visual of regex at 'rxrx' line 0]         [step: 18]
---

[download]

So hopefully you can see from rxrx output that zero-width assertions match from current position without advancing that pos()ition . "Look-behind matches text up to the current match position, look-ahead matches text following the current match position." The position is pos() an is advanced by matching patterns

So in the OPs , pos(0) tries to see ahead if '5' is 4 which its not, the bumps pos(1) and tries to see ahead if '4' is 4 and it is , but then it fails to match a W at pos(1) , because the look-ahead didn't advance pos()ition, and there is a 4 at pos(1) and not a W ..... :)

In reply to Re^2: zero-width assertions for extended regular expressions (rxrx) by Anonymous Monk
in thread zero-width assertions for extended regular expressions by johnrcomeau

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