add unescape function for conversion of \t, \n, \x1b etc. in <format> strings

src/Main/util.cxx

@@ -143,5 +143,57 @@ fgGetLowPass (double current, double target, double timeratio)
     return current;
 }
 
+
+string
+fgUnescape(const char *s)
+{
+    string r;
+    while (*s) {
+        if (*s != '\\') {
+            r += *s++;
+            continue;
+        }
+        if (!*++s)
+            break;
+        if (*s == '\\') {
+            r += '\\';
+        } else if (*s == 'n') {
+            r += '\n';
+        } else if (*s == 'r') {
+            r += '\r';
+        } else if (*s == 't') {
+            r += '\t';
+        } else if (*s == 'v') {
+            r += '\v';
+        } else if (*s == 'f') {
+            r += '\f';
+        } else if (*s == 'a') {
+            ;
+        } else if (*s == 'b') {
+            ;
+        } else if (*s == 'x') {
+            if (!*++s)
+                break;
+            int v = 0;
+            for (int i = 0; i < 2 && isxdigit(*s); i++, s++)
+                v = v * 16 + (isdigit(*s) ? *s - '0' : 10 + tolower(*s) - 'a');
+            r += v;
+            continue;
+
+        } else if (*s >= '0' && *s <= '7') {
+            int v = *s++ - '0';
+            for (int i = 0; i < 3 && *s >= '0' && *s <= '7'; i++, s++)
+                v = v * 8 + *s - '0';
+            r += v;
+            continue;
+
+        } else {
+            r += *s;
+        }
+        s++;
+    }
+    return r;
+}
+
 // end of util.cxx
 

src/Main/util.hxx

@@ -74,4 +74,13 @@ extern void fgExit (int status = 0);
 extern double fgGetLowPass (double current, double target, double timeratio);
 
 
+/**
+ * Unescape string.
+ *
+ * @param str String possibly containing escaped characters.
+ * @return string with escaped characters replaced by single character values.
+ */
+extern std::string fgUnescape (const char *str);
+
+
 #endif // __UTIL_HXX