The regex languages con be used for inline regex snippets like (?<number>\d+)[-_ ]\k<number> but it mainly adds itself to other languages such as:

JavaScript

Prism.languages.markup = {
	'comment': /<!--[\s\S]*?-->/,
	'prolog': /<\?[\s\S]+?\?>/,
	'doctype': {
		pattern: /<!DOCTYPE(?:[^>"'[\]]|"[^"]*"|'[^']*')+(?:\[(?:[^<"'\]]|"[^"]*"|'[^']*'|<(?!!--)|<!--(?:[^-]|-(?!->))*-->)*\]\s*)?>/i,
		greedy: true
	},
	'cdata': /<!\[CDATA\[[\s\S]*?]]>/i,
	'tag': {
		pattern: /<\/?(?!\d)[^\s>\/=$<%]+(?:\s(?:\s*[^\s>\/=]+(?:\s*=\s*(?:"[^"]*"|'[^']*'|[^\s'">=]+(?=[\s>]))|(?=[\s/>])))+)?\s*\/?>/i,
		greedy: true,
		inside: {
			'tag': {
				pattern: /^<\/?[^\s>\/]+/i,
				inside: {
					'punctuation': /^<\/?/,
					'namespace': /^[^\s>\/:]+:/
				}
			},
			'attr-value': {
				pattern: /=\s*(?:"[^"]*"|'[^']*'|[^\s'">=]+)/i,
				inside: {
					'punctuation': [
						/^=/,
						{
							pattern: /^(\s*)["']|["']$/,
							lookbehind: true
						}
					]
				}
			},
			'punctuation': /\/?>/,
			'attr-name': {
				pattern: /[^\s>\/]+/,
				inside: {
					'namespace': /^[^\s>\/:]+:/
				}
			}

		}
	},
	'entity': /&#?[\da-z]{1,8};/i
};