shell - Bash regex =~ operator

Question

What is the operator =~ called? Is it only used to compare the right side against the left side?

Why are double square brackets required when running a test?

ie. [[ $phrase =~ $keyword ]]

Thank you

Answer 1

1. What is the operator =~ called?

   I'm not sure it has a name. The bash documentation just calls it the =~ operator.

2. Is it only used to compare the right side against the left side?

   The right side is considered an extended regular expression. If the left side matches, the operator returns 0, and 1 otherwise.

3. Why are double square brackets required when running a test?

   Because =~ is an operator of the [[ expression ]] compound command.