S_parse_ident: Add flag to check for legality and not croak

When this flag is set, when an illegal identifier is found, instead of
croaking the function returns NULL.  This will allow future commits to
use this function when all that is desired is to determine if the
identifier is legal or not.

Author: khwilliamson

toke.c

@@ -173,6 +173,7 @@ static const char ident_var_zero_multi_digit[] = "Numeric variables with more th
 #define CHECK_DOLLAR                (1 << 2)
 #define IDFIRST_ONLY                (1 << 3)
 #define STOP_AT_FIRST_NON_DIGIT     (1 << 4)
+#define CHECK_ONLY                  (1 << 5)
 
 #ifdef DEBUGGING
 static const char* const lex_state_names[] = {
@@ -10334,7 +10335,13 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
      * identifier ends in the input.  If no identifier was found, the return
      * will be the the input 's' unchanged.
      *
-     * If the identifier is illegal, the function croaks.
+     * If the identifier is illegal, the function croaks unless this flag is
+     * passed in: */
+    const bool check_only = flags & CHECK_ONLY;
+
+    /* In this case NULL is returned instead of croaking, and the contents
+     * of '*d' are undefined.
+     *
      * The possible reasons for failure are:
      *  1) There is not enough room for the entire source identifier to be
      *     copied
@@ -10403,8 +10410,12 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
             } while (isDIGIT_A(*s));
 
             /* Leading zeros are not permitted */
-            if (is_zero && *d - digit_start > 1)
+            if (is_zero && *d - digit_start > 1) {
+                if (check_only) {
+                    return NULL;
+                }
                 croak(ident_var_zero_multi_digit);
+            }
 
             break;
         }
@@ -10453,6 +10464,10 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
     return (char *) s;
 
   too_long:
+    if (check_only) {
+        return NULL;
+    }
+
     croak("%s", ident_too_long);
 }