I searched for the meaning of these expressions but couldn't understand the exact difference between them.
This is what they say:
?:
Match expression but do not capture it.?=
Match a suffix but exclude it from capture.?!
Match if the suffix is absent.I tried using these in simple RegEx and got similar results for all.
For example: the following 3 expressions give very similar results.
[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+(?!\.[a-zA-Z0-9]+)*
[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+(?=\.[a-zA-Z0-9]+)*
[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9]+)*
The difference between ?=
and ?!
is that the former requires the given expression to match and the latter requires it to not match. For example a(?=b)
will match the "a" in "ab", but not the "a" in "ac". Whereas a(?!b)
will match the "a" in "ac", but not the "a" in "ab".
The difference between ?:
and ?=
is that ?=
excludes the expression from the entire match while ?:
just doesn't create a capturing group. So for example a(?:b)
will match the "ab" in "abc", while a(?=b)
will only match the "a" in "abc". a(b)
would match the "ab" in "abc" and create a capture containing the "b".