What does the given string mean in Java String?

Question

Java

There was a need to prescribe IP Address in edittext. It was decided to search and find a solution everyone knows where.
The solution is concise and understandable for me, but there was a small question on the following line:

str.matches("^\\d{1,3}(\\.(\\d{1,3}(\\.(\\d{1,3}(\\.(\\d{1,3})?)?)?)?)?)?"))

Actually the question is what does ^, \\, d{1,3}, ? and other characters?
A clear description of all known where, could not be found.

Answer 1

This is a regular expression