zero width token?

Question

I want to ask a very basic question about token,
while reading about regex,the book tag caret(^) as a zero width token, can you please tell me what actually it means by zero width?

Answer 1

It means that it matches without consuming any characters. It is simply a positional assertion ("must be at the start of the line"). Another example is zero-width look-ahead and look-behind assertions. For instance, the Perl regex /abc(?=123)/ matches the sequence abc only if it is followed by the sequence 123, but it doesn't actually consume the 123.