Regular Expressions 101

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/
(<\?)(.*?)(?:$|\?>)
/
isg
1st Capturing Group
(<\?)
< matches the character < with index 6010 (3C16 or 748) literally (case insensitive)
\? matches the character ? with index 6310 (3F16 or 778) literally (case insensitive)
2nd Capturing Group
(.*?)
.
matches any character
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
Non-capturing group
(?:$|\?>)
1st Alternative
$
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
2nd Alternative
\?>
\? matches the character ? with index 6310 (3F16 or 778) literally (case insensitive)
> matches the character > with index 6210 (3E16 or 768) literally (case insensitive)
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
s modifier: single line. Dot matches newline characters
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

/
/
isg

Test String