what is the difference between ?:, ?! and ?= in regex?

Question

I searched for the meaning of these expressions but couldn\'t understand the exact difference between them. This is what they say:

• ?: Match expres

Answer 1

?:  is for non capturing group
?=  is for positive look ahead
?!  is for negative look ahead
?<= is for positive look behind
?

Please check here: http://www.regular-expressions.info/lookaround.html for very good tutorial and examples on lookahead in regular expressions.