Regular Expressions 101

Explanation
Pro

Group comment(?<comment>)

<!---the literal text <!---

Condition(?(?=<!---)\g<comment>|.)*?

*?repeats the group contents below any number of times:

Uses the first branch if the lookaround matches

Positive Lookahead(?=<!---)

<!---the literal text <!---

If the condition is true, match this branch:\g<comment>

\g<comment>calls group comment as a subroutine

Otherwise, match this branch:.

.any character

--->the literal text --->

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

s modifier: single line. Allows a dot to match line terminators

Detailed match information will be displayed here automatically.

25 tokens

(?<comment><!---(?(?=<!---)\g<comment>|.)*?--->)

New

foo bar

<!---
multiline <!--- nested --->
comment <!--- comment --->
--->

baz

foo bar

<!---
multiline <!--- nested --->
comment <!--- comment --->
--->

baz