LibRegex+LibJS: Avoid searching for more than one match in JS RegExps

All of JS's regular expression APIs only want a single match, so avoid
trying to produce more (which will be discarded anyway).

Tests/LibRegex/Regex.cpp

@@ -990,3 +990,15 @@ TEST_CASE(negative_lookahead)
         EXPECT_EQ(re.match(":foobar").success, true);
     }
 }
+
+TEST_CASE(single_match_flag)
+{
+    {
+        // Ensure that only a single match is produced and nothing past that.
+        Regex<ECMA262> re("[\\u0008-\\uffff]"sv, ECMAScriptFlags::Global | (ECMAScriptFlags)regex::AllFlags::SingleMatch);
+        auto result = re.match("ABC");
+        EXPECT_EQ(result.success, true);
+        EXPECT_EQ(result.matches.size(), 1u);
+        EXPECT_EQ(result.matches.first().view.to_string(), "A"sv);
+    }
+}

Userland/Libraries/LibC/regex.h

@@ -81,10 +81,11 @@ enum __RegexAllFlags {
     __Regex_Sticky = __Regex_Global << 11,                   // Force the pattern to only match consecutive matches from where the previous match ended.
     __Regex_Multiline = __Regex_Global << 12,                // Handle newline characters. Match each line, one by one.
     __Regex_SkipTrimEmptyMatches = __Regex_Global << 13,     // Do not remove empty capture group results.
-    __Regex_Internal_Stateful = __Regex_Global << 14,        // Internal flag; enables stateful matches.
-    __Regex_Internal_BrowserExtended = __Regex_Global << 15, // Internal flag; enable browser-specific ECMA262 extensions.
-    __Regex_Internal_ConsiderNewline = __Regex_Global << 16, // Internal flag; allow matchers to consider newlines as line separators.
-    __Regex_Last = __Regex_SkipTrimEmptyMatches
+    __Regex_SingleMatch = __Regex_Global << 14,              // Stop after acquiring a single match.
+    __Regex_Internal_Stateful = __Regex_Global << 15,        // Internal flag; enables stateful matches.
+    __Regex_Internal_BrowserExtended = __Regex_Global << 16, // Internal flag; enable browser-specific ECMA262 extensions.
+    __Regex_Internal_ConsiderNewline = __Regex_Global << 17, // Internal flag; allow matchers to consider newlines as line separators.
+    __Regex_Last = __Regex_SingleMatch
 };
 
 // Values for the cflags parameter to the regcomp() function:

Userland/Libraries/LibJS/Runtime/RegExpObject.h

@@ -25,7 +25,12 @@ class RegExpObject : public Object {
 public:
     // JS regexps are all 'global' by default as per our definition, but the "global" flag enables "stateful".
     // FIXME: Enable 'BrowserExtended' only if in a browser context.
-    static constexpr regex::RegexOptions<ECMAScriptFlags> default_flags { (regex::ECMAScriptFlags)regex::AllFlags::Global | (regex::ECMAScriptFlags)regex::AllFlags::SkipTrimEmptyMatches | regex::ECMAScriptFlags::BrowserExtended };
+    static constexpr regex::RegexOptions<ECMAScriptFlags> default_flags {
+        (regex::ECMAScriptFlags)regex::AllFlags::SingleMatch
+        | (regex::ECMAScriptFlags)regex::AllFlags::Global
+        | (regex::ECMAScriptFlags)regex::AllFlags::SkipTrimEmptyMatches
+        | regex::ECMAScriptFlags::BrowserExtended
+    };
 
     static RegExpObject* create(GlobalObject&);
     static RegExpObject* create(GlobalObject&, Regex<ECMA262> regex, String pattern, String flags);

Userland/Libraries/LibRegex/RegexMatcher.cpp

@@ -183,6 +183,8 @@ RegexResult Matcher<Parser>::match(Vector<RegexStringView> const& views, Optiona
     if (input.regex_options.has_flag_set(AllFlags::Internal_Stateful))
         continue_search = false;
 
+    auto single_match_only = input.regex_options.has_flag_set(AllFlags::SingleMatch);
+
     for (auto const& view : views) {
         if (lines_to_skip != 0) {
             ++input.line;
@@ -276,6 +278,8 @@ RegexResult Matcher<Parser>::match(Vector<RegexStringView> const& views, Optiona
 
                     bool has_zero_length = state.string_position == view_index;
                     view_index = state.string_position - (has_zero_length ? 0 : 1);
+                    if (single_match_only)
+                        break;
                     continue;
                 }
                 if (input.regex_options.has_flag_set(AllFlags::Internal_Stateful)) {

Userland/Libraries/LibRegex/RegexOptions.h

@@ -33,6 +33,7 @@ enum class AllFlags {
     Sticky = __Regex_Sticky,                                     // Force the pattern to only match consecutive matches from where the previous match ended.
     Multiline = __Regex_Multiline,                               // Handle newline characters. Match each line, one by one.
     SkipTrimEmptyMatches = __Regex_SkipTrimEmptyMatches,         // Do not remove empty capture group results.
+    SingleMatch = __Regex_SingleMatch,                           // Stop after acquiring a single match.
     Internal_Stateful = __Regex_Internal_Stateful,               // Make global matches match one result at a time, and further match() calls on the same instance continue where the previous one left off.
     Internal_BrowserExtended = __Regex_Internal_BrowserExtended, // Only for ECMA262, Enable the behaviors defined in section B.1.4. of the ECMA262 spec.
     Internal_ConsiderNewline = __Regex_Internal_ConsiderNewline, // Only for ECMA262, Allow multiline matches to consider newlines as line boundaries.