Can you please be more precise where I should use $[]? Because in all examples I’ve gone through uses ${}
Also curly brackets { } are general repetition quantifiers. They specify a minimum and maximum number of permitted matches. And here I’m matching ‘DB_EXISTS’ and ‘EXTEN’
Expressions are combinations of variables, operators, and values that you string together to produce a result. An expression can test values, alter strings, or perform mathematical calculations.
Everything contained inside a bracket pair prefixed by a $ (like $[this]) is considered as an expression and it is evaluated.