What does this regular expression mean?

Question

In a recent interview I was asked to decipher this regex

^\^[^^]

Can you please help me with it. Also please provide some links where I can learn regex for interviews.

Answer 1

It matches strings that begin with ^ followed by any character other than ^.

So it would match:

^foo
^b

but not

foo
^^b

Explanation:

Caret (^) is a regex meta character with two different meanings:

Outside the character class(1st use in your regex) it works as start anchor.

Inside the character class it acts like negator if used as the first character of the character class(3rd use in your regex).

Preceding a regex with \ escapes it (makes it non-special). The 2nd use of ^ in your regex is escaped and it matches a literal ^ in the string.

Inside a character class a ^ which is not the first character of the character class is treated literally. So the 4th use in your regex is a literal ^.

Some more examples to make it clear:

• ^a         : Matches string beginning with a
• ^ab       : Matches string beginning with a followed by b
• [a]       : Matches a string which has an a
• [^a]     : Matches a string which does not have an a
• ^a[^a] : Matches a string beginning with an a followed by any character other than a.

Answer 2

I'm testing this regex here however it does not seem to be valid.
The first ^ denotes the start of the line.
The first \ escapes the following \.
Thus the second "^" is not escaped Finally the first caret inside the square brackets [^ acts as the negation and second one ^] is not escaped as a result is not valid.

IMHO the correct regexp should be ^\^[^\^]
Guys, kindly confirm. Many thanks