test look-behind, '}' so succeeds
test look-ahead, '[' also succeeds
code block encountered, execute
advance pointer one place

[download]

That last step is wrong. It should be 

advance pointer the length of the match

[download]

The length of the match is zero in the case where the lookahead and lookbehind is used. Since the pointer is not advanced, the regexp matches everything twice. Only a final check prevents the regexp from returning an identical match. 

test look-behind, '}' so succeeds
test look-ahead, '[' also succeeds
code block encountered, execute
same match? no, continue
advance pointer the length of the match (0)

test look-behind, '}' so succeeds
test look-ahead, '[' also succeeds
code block encountered, execute
same match? yes, backtrack

[download]

More on this in Re: Regex code block executes twice per match using look-arounds.

In reply to Re^3: Regex code block executes twice per match using look-arounds by ikegami
in thread Regex code block executes twice per match using look-arounds by johngg

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