Regular Expressions 101

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/
(?:a(?=b))?(?:b(?=c))?(?:c(?=d))?(?:d(?=e))?
/
g
Non-capturing group
(?:a(?=b))?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
a matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
Positive Lookahead
(?=b)
Assert that the Regex below matches
b matches the character b with index 9810 (6216 or 1428) literally (case sensitive)
Non-capturing group
(?:b(?=c))?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
b matches the character b with index 9810 (6216 or 1428) literally (case sensitive)
Positive Lookahead
(?=c)
Assert that the Regex below matches
c matches the character c with index 9910 (6316 or 1438) literally (case sensitive)
Non-capturing group
(?:c(?=d))?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
c matches the character c with index 9910 (6316 or 1438) literally (case sensitive)
Positive Lookahead
(?=d)
Assert that the Regex below matches
d matches the character d with index 10010 (6416 or 1448) literally (case sensitive)
Non-capturing group
(?:d(?=e))?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
d matches the character d with index 10010 (6416 or 1448) literally (case sensitive)
Positive Lookahead
(?=e)
Assert that the Regex below matches
e matches the character e with index 10110 (6516 or 1458) literally (case sensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
g

Test String