add getACodepoint to the shared Strings library, and use it in NfaUtils

shared/regex/codeql/regex/nfa/NfaUtils.qll

@@ -164,17 +164,17 @@ module Make<RegexTreeViewSig TreeImpl> {
     /** An input symbol corresponding to character `c`. */
     Char(string c) {
       c =
-        getCodepointAt(any(RegexpCharacterConstant cc |
+        getACodepoint(any(RegexpCharacterConstant cc |
             cc instanceof RelevantRegExpTerm and
             not isIgnoreCase(cc.getRootTerm())
-          ).getValue(), _)
+          ).getValue())
       or
       // normalize everything to lower case if the regexp is case insensitive
       c =
         any(RegexpCharacterConstant cc, string char |
           cc instanceof RelevantRegExpTerm and
           isIgnoreCase(cc.getRootTerm()) and
-          char = getCodepointAt(cc.getValue(), _)
+          char = getACodepoint(cc.getValue())
         |
           char.toLowerCase()
         )
@@ -370,7 +370,7 @@ module Make<RegexTreeViewSig TreeImpl> {
     string getARelevantChar() {
       exists(asciiPrintable(result))
       or
-      exists(RegexpCharacterConstant c | result = getCodepointAt(c.getValue(), _))
+      exists(RegexpCharacterConstant c | result = getACodepoint(c.getValue()))
       or
       classEscapeMatches(_, result)
     }
@@ -1258,7 +1258,7 @@ module Make<RegexTreeViewSig TreeImpl> {
        * Gets a `char` that occurs in a `pump` string.
        */
       private string getAProcessChar() {
-        result = getCodepointAt(any(string s | isReDoSCandidate(_, s)), _)
+        result = getACodepoint(any(string s | isReDoSCandidate(_, s)))
       }
     }
 

shared/util/codeql/util/Strings.qll

@@ -63,6 +63,12 @@ string getCodepointAt(string s, int i) {
     )
 }
 
+/**
+ * Gets any unicode character that appears in `s`.
+ */
+bindingset[s]
+string getACodepoint(string s) { result = s.codePointAt(_).toUnicode() }
+
 /**
  * Gets the number of unicode codepoints in `s` not counting unpaired surrogates.
  */