Add parsing of hex literals in strings

With this patch it is possible to give character values by using
the common '\xHH' escape where H is a hexadecimal digit.

src/scanner.c

@@ -1869,6 +1869,13 @@ void GetStr ( void )
       else if ( *TLS(In) == '>'  )  TLS(Value)[i] = '\01';
       else if ( *TLS(In) == '<'  )  TLS(Value)[i] = '\02';
       else if ( *TLS(In) == 'c'  )  TLS(Value)[i] = '\03';
+      else if ( *TLS(In) == 'x'  ) {
+        GET_CHAR(); a = *TLS(In); GET_CHAR(); b = *TLS(In);
+        if (!( IsHexDigit(a) && IsHexDigit(b) )) {
+          SyntaxError("Expecting two hexadecimal digits after \\x in string");
+        }
+        TLS(Value)[i] = CharHexDigit(a)*16 + CharHexDigit(b);
+      }
       else if ( IsDigit( *TLS(In) ) ) {
         a = *TLS(In); GET_CHAR(); b = *TLS(In); GET_CHAR(); c = *TLS(In);
         if (!( IsDigit(b) && IsDigit(c) )){
@@ -2054,6 +2061,18 @@ void GetChar ( void )
     else if ( *TLS(In) == '>'  )  TLS(Value)[0] = '\01';
     else if ( *TLS(In) == '<'  )  TLS(Value)[0] = '\02';
     else if ( *TLS(In) == 'c'  )  TLS(Value)[0] = '\03';
+    else if ( *TLS(In) == 'x'  ) {
+      GET_CHAR(); c = *TLS(In);
+      if (!IsHexDigit(c)) {
+        SyntaxError("Expecting hexadecimal digit in character constant");
+      } 
+      TLS(Value)[0] = CharHexDigit(c) * 16;
+      GET_CHAR(); c = *TLS(In);
+      if (!IsHexDigit(c)) {
+        SyntaxError("Expecting hexadecimal digit in character constant");
+      } 
+      TLS(Value)[0] += CharHexDigit(c);
+    }
     else if ( *TLS(In) >= '0' && *TLS(In) <= '7' ) {
       /* escaped three digit octal numbers are allowed in input */
       c = 64 * (*TLS(In) - '0');

src/system.h

@@ -630,6 +630,16 @@ extern UInt SyTimeChildrenSys ( void );
 */
 #define IsDigit(ch)     (isdigit((unsigned int)ch))
 
+
+/****************************************************************************
+**
+*F  IsHexDigit( <ch> ) . . . . . . . . . . . . . . .  is a character a digit
+**
+**  'IsDigit' returns 1 if its character argument is a digit from the ranges
+**  '0..9' or 'a..f' and 0 otherwise.
+*/
+#define IsHexDigit(ch)     (isxdigit((unsigned int)ch))
+
 /****************************************************************************
 **
 *F  IsSpace( <ch> ) . . . . . . . . . . . . . . . .is a character whitespace
@@ -640,6 +650,21 @@ extern UInt SyTimeChildrenSys ( void );
 #define IsSpace(ch)     (isspace((unsigned int)ch))
 
 
+/****************************************************************************
+**
+*F  IntHexDigit( <ch> ) . . . . . . . . . . turn a single hex digit into int 
+**
+*/
+inline Char CharHexDigit( const Char ch ) {
+    if (ch >= 'a') {
+        return (ch - 'a' + 10);
+    } else if (ch >= 'A') {
+        return (ch - 'A' + 10);
+    } else {
+        return (ch - '0');
+    }
+};
+
 /****************************************************************************
 **
 *F  SyIntString( <string> ) . . . . . . . . extract a C integer from a string