Editorial: Extract operation 'ParsePattern' (tc39#1866)

... from common code in IsValidRegularExpressionLiteral and RegExpInitialize.

(This was originally presented as a series of 12 small refactorings.
For more info, see the PR page.)

spec.html

@@ -12971,12 +12971,13 @@ <h1>Static Semantics: IsValidRegularExpressionLiteral ( _literal_ )</h1>
         <emu-alg>
           1. Assert: _literal_ is a |RegularExpressionLiteral|.
           1. If FlagText of _literal_ contains any code points other than `g`, `i`, `m`, `s`, `u`, or `y`, or if it contains the same code point more than once, return *false*.
-          1. Let _P_ be BodyText of _literal_.
-          1. If FlagText of _literal_ contains `u`, then
-            1. Parse _P_ using the grammars in <emu-xref href="#sec-patterns"></emu-xref>. The goal symbol for the parse is |Pattern[+U, +N]|. If _P_ did not conform to the grammar, if any elements of _P_ were not matched by the parse, or if any Early Error conditions exist, return *false*. Otherwise, return *true*.
-          1. Let _stringValue_ be UTF16Encode(_P_).
-          1. Let _pText_ be the sequence of code points resulting from interpreting each of the 16-bit elements of _stringValue_ as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
-          1. Parse _pText_ using the grammars in <emu-xref href="#sec-patterns"></emu-xref>. The goal symbol for the parse is |Pattern[~U, ~N]|. If the result of parsing contains a |GroupName|, reparse with the goal symbol |Pattern[~U, +N]|. If _pText_ did not conform to the grammar, if any elements of _pText_ were not matched by the parse, or if any Early Error conditions exist, return *false*. Otherwise, return *true*.
+          1. Let _patternText_ be BodyText of _literal_.
+          1. If FlagText of _literal_ contains `u`, let _u_ be *true*; else let _u_ be *false*.
+          1. If _u_ is *false*, then
+            1. Let _stringValue_ be UTF16Encode(_patternText_).
+            1. Set _patternText_ to the sequence of code points resulting from interpreting each of the 16-bit elements of _stringValue_ as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
+          1. Let _parseResult_ be ParsePattern(_patternText_, _u_).
+          1. If _parseResult_ is a Parse Node, return *true*; else return *false*.
         </emu-alg>
       </emu-clause>
 
@@ -32488,23 +32489,37 @@ <h1>Runtime Semantics: RegExpInitialize ( _obj_, _pattern_, _flags_ )</h1>
             1. If _flags_ is *undefined*, let _F_ be the empty String.
             1. Else, let _F_ be ? ToString(_flags_).
             1. If _F_ contains any code unit other than *"g"*, *"i"*, *"m"*, *"s"*, *"u"*, or *"y"* or if it contains the same code unit more than once, throw a *SyntaxError* exception.
-            1. If _F_ contains *"u"*, let _BMP_ be *false*; else let _BMP_ be *true*.
-            1. If _BMP_ is *true*, then
-              1. Let _pText_ be the sequence of code points resulting from interpreting each of the 16-bit elements of _P_ as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
-              1. Parse _pText_ using the grammars in <emu-xref href="#sec-patterns"></emu-xref>. The goal symbol for the parse is |Pattern[~U, ~N]|. If the result of parsing contains a |GroupName|, reparse with the goal symbol |Pattern[~U, +N]| and use this result instead. Throw a *SyntaxError* exception if _pText_ did not conform to the grammar, if any elements of _pText_ were not matched by the parse, or if any Early Error conditions exist.
-              1. Let _patternCharacters_ be a List whose elements are the code unit elements of _P_.
+            1. If _F_ contains *"u"*, let _u_ be *true*; else let _u_ be *false*.
+            1. If _u_ is *true*, then
+              1. Let _patternText_ be ! UTF16DecodeString(_P_).
+              1. Let _patternCharacters_ be a List whose elements are the code points of _patternText_.
             1. Else,
-              1. Let _pText_ be ! UTF16DecodeString(_P_).
-              1. Parse _pText_ using the grammars in <emu-xref href="#sec-patterns"></emu-xref>. The goal symbol for the parse is |Pattern[+U, +N]|. Throw a *SyntaxError* exception if _pText_ did not conform to the grammar, if any elements of _pText_ were not matched by the parse, or if any Early Error conditions exist.
-              1. Let _patternCharacters_ be a List whose elements are the code points of _pText_.
+              1. Let _patternText_ be the result of interpreting each of _P_'s 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
+              1. Let _patternCharacters_ be a List whose elements are the code unit elements of _P_.
+            1. Let _parseResult_ be ParsePattern(_patternText_, _u_).
+            1. If _parseResult_ is a non-empty List of *SyntaxError* objects, throw a *SyntaxError* exception.
+            1. Assert: _parseResult_ is a Parse Node for |Pattern|.
             1. Set _obj_.[[OriginalSource]] to _P_.
             1. Set _obj_.[[OriginalFlags]] to _F_.
-            1. Set _obj_.[[RegExpMatcher]] to the Abstract Closure that evaluates the above parse by applying the semantics provided in <emu-xref href="#sec-pattern-semantics"></emu-xref> using _patternCharacters_ as the pattern's List of |SourceCharacter| values and _F_ as the flag parameters.
+            1. Set _obj_.[[RegExpMatcher]] to the Abstract Closure that evaluates _parseResult_ by applying the semantics provided in <emu-xref href="#sec-pattern-semantics"></emu-xref> using _patternCharacters_ as the pattern's List of |SourceCharacter| values and _F_ as the flag parameters.
             1. Perform ? Set(_obj_, *"lastIndex"*, 0, *true*).
             1. Return _obj_.
           </emu-alg>
         </emu-clause>
 
+        <emu-clause id="sec-parsepattern" aoid="ParsePattern">
+          <h1>Static Semantics: ParsePattern ( _patternText_, _u_ )</h1>
+          <p>The abstract operation ParsePattern takes arguments _patternText_ (a sequence of Unicode code points) and _u_ (a Boolean). It performs the following steps when called:</p>
+          <emu-alg>
+            1. If _u_ is *true*, then
+              1. Parse _patternText_ using the grammars in <emu-xref href="#sec-patterns"></emu-xref>. The goal symbol for the parse is |Pattern[+U, +N]|.
+            1. Else,
+              1. Parse _patternText_ using the grammars in <emu-xref href="#sec-patterns"></emu-xref>. The goal symbol for the parse is |Pattern[~U, ~N]|. If the result of parsing contains a |GroupName|, reparse with the goal symbol |Pattern[~U, +N]| and use this result instead.
+            1. If _patternText_ did not conform to the grammar, or any elements of _patternText_ were not matched by the parse, or any Early Error conditions exist, return a List of one or more *SyntaxError* objects representing the parsing errors and/or early errors.
+            1. Otherwise, return the Parse Node resulting from the parse.
+          </emu-alg>
+        </emu-clause>
+
         <emu-clause id="sec-regexpcreate" aoid="RegExpCreate">
           <h1>Runtime Semantics: RegExpCreate ( _P_, _F_ )</h1>
           <p>The abstract operation RegExpCreate takes arguments _P_ and _F_. It performs the following steps when called:</p>