LibRegex: Allow separately parsing patterns and creating Regex objects

Adds a static method to parse a regex pattern and return the result, and
a constructor to accept a parse result. This is to allow LibJS to parse
the pattern string of a RegExpLiteral once and hand off regex objects
any number of times thereafter.

Userland/Libraries/LibRegex/RegexMatcher.cpp

@@ -18,6 +18,15 @@ namespace regex {
 static RegexDebug s_regex_dbg(stderr);
 #endif
 
+template<class Parser>
+regex::Parser::Result Regex<Parser>::parse_pattern(StringView pattern, typename ParserTraits<Parser>::OptionsType regex_options)
+{
+    regex::Lexer lexer(pattern);
+
+    Parser parser(lexer, regex_options);
+    return parser.parse();
+}
+
 template<class Parser>
 Regex<Parser>::Regex(String pattern, typename ParserTraits<Parser>::OptionsType regex_options)
     : pattern_value(move(pattern))
@@ -31,6 +40,15 @@ Regex<Parser>::Regex(String pattern, typename ParserTraits<Parser>::OptionsType
         matcher = make<Matcher<Parser>>(this, regex_options);
 }
 
+template<class Parser>
+Regex<Parser>::Regex(regex::Parser::Result parse_result, String pattern, typename ParserTraits<Parser>::OptionsType regex_options)
+    : pattern_value(move(pattern))
+    , parser_result(move(parse_result))
+{
+    if (parser_result.error == regex::Error::NoError)
+        matcher = make<Matcher<Parser>>(this, regex_options);
+}
+
 template<class Parser>
 Regex<Parser>::Regex(Regex&& regex)
     : pattern_value(move(regex.pattern_value))

Userland/Libraries/LibRegex/RegexMatcher.h

@@ -80,7 +80,10 @@ public:
     OwnPtr<Matcher<Parser>> matcher { nullptr };
     mutable size_t start_offset { 0 };
 
+    static regex::Parser::Result parse_pattern(StringView pattern, typename ParserTraits<Parser>::OptionsType regex_options = {});
+
     explicit Regex(String pattern, typename ParserTraits<Parser>::OptionsType regex_options = {});
+    Regex(regex::Parser::Result parse_result, String pattern, typename ParserTraits<Parser>::OptionsType regex_options = {});
     ~Regex() = default;
     Regex(Regex&&);
     Regex& operator=(Regex&&);