+/*
+=for apidoc grok_hex
+
+converts a string representing a hex number to numeric form.
+
+On entry I<start> and I<*len> give the string to scan, I<*flags> gives
+conversion flags, and I<result> should be NULL or a pointer to an NV.
+The scan stops at the end of the string, or the first invalid character.
+Unless C<PERL_SCAN_SILENT_ILLDIGIT> is set in I<*flags>, encountering an
+invalid character will also trigger a warning.
+On return I<*len> is set to the length of the scanned string,
+and I<*flags> gives output flags.
+
+If the value is <= UV_MAX it is returned as a UV, the output flags are clear,
+and nothing is written to I<*result>. If the value is > UV_MAX C<grok_hex>
+returns UV_MAX, sets C<PERL_SCAN_GREATER_THAN_UV_MAX> in the output flags,
+and writes the value to I<*result> (or the value is discarded if I<result>
+is NULL).
+
+The hex number may optionally be prefixed with "0x" or "x" unless
+C<PERL_SCAN_DISALLOW_PREFIX> is set in I<*flags> on entry. If
+C<PERL_SCAN_ALLOW_UNDERSCORES> is set in I<*flags> then the hex
+number may use '_' characters to separate digits.
+
+=cut
+ */
+
+UV
+Perl_grok_hex(pTHX_ const char *start, STRLEN *len_p, I32 *flags, NV *result) {
+ dVAR;
+ const char *s = start;
+ STRLEN len = *len_p;
+ UV value = 0;
+ NV value_nv = 0;
+
+ const UV max_div_16 = UV_MAX / 16;
+ const bool allow_underscores = (bool)(*flags & PERL_SCAN_ALLOW_UNDERSCORES);
+ bool overflowed = FALSE;
+
+ if (!(*flags & PERL_SCAN_DISALLOW_PREFIX)) {
+ /* strip off leading x or 0x.
+ for compatibility silently suffer "x" and "0x" as valid hex numbers.
+ */
+ if (len >= 1) {
+ if (s[0] == 'x') {
+ s++;
+ len--;
+ }
+ else if (len >= 2 && s[0] == '0' && s[1] == 'x') {
+ s+=2;
+ len-=2;
+ }
+ }
+ }