Index: src/obj_text.c
===================================================================
--- src/obj_text.c	(revision 35640)
+++ src/obj_text.c	(revision 35641)
@@ -1332,7 +1332,31 @@
 	return rnd_round((double)h * scy);
 }
 
+int pcb_text_invalid_chars(pcb_board_t *pcb, pcb_font_t *FontPtr, pcb_text_t *Text)
+{
+	unsigned char *s, *rendered;
+	int ctr = 0;
+	pcb_symbol_t *symbol;
 
+	if (FontPtr == NULL)
+		FontPtr = pcb_font(pcb, Text->fid, 1);
+
+	symbol = FontPtr->Symbol;
+
+	rendered = pcb_text_render_str(Text);
+	if (rendered == NULL)
+		return 0;
+
+	for(s = rendered; *s != '\0'; s++)
+		if ((*s > PCB_MAX_FONTPOSITION) || (!symbol[*s].Valid))
+			ctr++;
+
+	pcb_text_free_str(Text, rendered);
+
+	return ctr;
+}
+
+
 RND_INLINE void cheap_text_line(rnd_hid_gc_t gc, pcb_xform_mx_t mx, rnd_coord_t x1, rnd_coord_t y1, rnd_coord_t x2, rnd_coord_t y2, rnd_coord_t xordx, rnd_coord_t xordy)
 {
 	rnd_coord_t tx1, ty1, tx2, ty2;
Index: src/obj_text.h
===================================================================
--- src/obj_text.h	(revision 35640)
+++ src/obj_text.h	(revision 35641)
@@ -125,6 +125,9 @@
 /* Return the minimum width the text object needs to have on a specific layer */
 rnd_coord_t pcb_text_min_thickness(const pcb_layer_t *layer);
 
+/* Return the number of invalid characters (with the given font) the text contains */
+int pcb_text_invalid_chars(pcb_board_t *pcb, pcb_font_t *FontPtr, pcb_text_t *Text);
+
 /* hash and eq */
 int pcb_text_eq(const pcb_host_trans_t *tr1, const pcb_text_t *t1, const pcb_host_trans_t *tr2, const pcb_text_t *t2);
 unsigned int pcb_text_hash(const pcb_host_trans_t *tr, const pcb_text_t *t);