Which form supports regex matching within a conditional expression, enabling patterns like a string matching a regex?

• A. [[ expression ]]
• B. [ expression ]
• C. test
• D. grep

Answer: (A)

Using the double-bracket conditional [[ ... ]] lets you test a string against a regular expression directly, by using the =~ operator. When you write something like [[ string =~ regex ]], Bash checks if the string matches the provided regex. If it does, the condition is true and the match details are stored in the special array BASH_REMATCH, with the whole match in BASH_REMATCH[0] and any captured groups in subsequent elements. This makes it straightforward to validate patterns such as a string matching a regex without invoking external tools.

For example, if [[ $input =~ ^[A-Za-z]+$ ]]; then echo "letters only"; fi

The single-bracket form relies on the test command and does not support regex matching with =~. Grep is a separate tool that could be used outside a conditional expression to test a string, but it requires running another process rather than evaluating the condition directly.