How to interpret this regular expression

<code>    const VARIABLE_REGEX = <<<'REGEX'
\{
    \s* ([a-zA-Z_][a-zA-Z0-9_-]*) \s*
    (?:
        : \s* ([^{}]*(?:\{(?-1)\}[^{}]*)*)
    )?
\}
REGEX;</code>

Reply content:

<code>    const VARIABLE_REGEX = <<<'REGEX'
\{
    \s* ([a-zA-Z_][a-zA-Z0-9_-]*) \s*
    (?:
        : \s* ([^{}]*(?:\{(?-1)\}[^{}]*)*)
    )?
\}
REGEX;</code>

You will need to use this https://www.debuggex.com