diff --git a/src/frontend/logicexp.c b/src/frontend/logicexp.c
index 614f4a57a..7ca949d82 100644
--- a/src/frontend/logicexp.c
+++ b/src/frontend/logicexp.c
@@ -353,10 +353,14 @@ static int lexer_scan(LEXER lx)
             return c;
         else if (lex_ident(c)) {
             size_t i = 0;
+            BOOL need_gt1 = FALSE;
             if (c == '+') { // an identifier does not begin with '+'
                 lx->lexer_buf[0] = (char) c;
                 lx->lexer_buf[1] = '\0';
                 return LEX_OTHER;
+            } else if (c == '_' || c == '/' || c == '-') {
+                // these need to be followed by at least one more ident
+                need_gt1 = TRUE;
             }
             while (lex_ident(c)) {
                 if (i >= lx->lexer_blen) {
@@ -368,6 +372,10 @@ static int lexer_scan(LEXER lx)
                 i++;
                 c = lexer_getchar(lx);
             }
+            if (i == 1 && need_gt1) {
+                lx->lexer_buf[1] = '\0';
+                return LEX_OTHER;
+            }
             if (i >= lx->lexer_blen) {
                 lx->lexer_blen *= 2;
                 lx->lexer_buf =
@@ -733,19 +741,37 @@ static int infix_to_postfix(char* infix, DSTRING * postfix_p)
     LEXER lx;
     NAME_ENTRY nlist = NULL, entry = NULL;
     int status = 0;
+    int lparen_count = 0, rparen_count = 0;
 
     lx = new_lexer(infix);
     stack.top = -1;
     nlist = add_name_entry("first", NULL);
     ds_clear(postfix_p);
     while ( ( ltok = lexer_scan(lx) ) != 0 ) { // start while ltok loop
+        if (ltok == '(') { lparen_count++; }
+        if (ltok == ')') { rparen_count++; }
         if (ltok == LEX_ID && last_tok == LEX_ID) {
-            fprintf(stderr, "ERROR no gate operator between two identifiers\n");
+            fprintf(stderr, "ERROR (1) no gate operator between two identifiers\n");
             status = 1;
             goto err_return;
         }
         if (lex_gate_op(ltok) && lex_gate_op(last_tok)) {
-            fprintf(stderr, "ERROR two consecutive gate operators\n");
+            fprintf(stderr, "ERROR (2) two consecutive gate operators\n");
+            status = 1;
+            goto err_return;
+        }
+        if (last_tok == '~' && ltok != LEX_ID && ltok != '(') {
+            fprintf(stderr, "ERROR (3) \'~\' is not followed by an ID or lparen\n");
+            status = 1;
+            goto err_return;
+        }
+        if (ltok == '~' && (last_tok == LEX_ID || last_tok == ')')) {
+            fprintf(stderr, "ERROR (4) \'~\' follows an ID or rparen\n");
+            status = 1;
+            goto err_return;
+        }
+        if (ltok == ')' && (lex_gate_op(last_tok) || last_tok == '~')) {
+            fprintf(stderr, "ERROR (5) incomplete infix sub-expression\n");
             status = 1;
             goto err_return;
         }
@@ -775,6 +801,7 @@ static int infix_to_postfix(char* infix, DSTRING * postfix_p)
                 next_tok = lexer_scan(lx);
                 if (next_tok == LEX_ID) {
                     ds_cat_printf(postfix_p, " tilde_%s", lx->lexer_buf);
+                    last_tok = next_tok;
                     continue;  // while ltok loop
                 } else {
                     lexer_back_one(lx);
@@ -787,12 +814,22 @@ static int infix_to_postfix(char* infix, DSTRING * postfix_p)
             entry = add_name_entry(tokstr, nlist);
             if (push(&stack, entry->name)) goto err_return;
         } else {
-            fprintf(stderr, "ERROR unexpected infix token %d \'%s\'\n",
+            fprintf(stderr, "ERROR (6) unexpected infix token %d \'%s\'\n",
                 ltok, lx->lexer_buf);
             status = 1;
             goto err_return;
         }
     } // end while ltok loop
+    if (lex_gate_op(last_tok) || last_tok == '~') {
+        fprintf(stderr, "ERROR (7) incomplete infix expression\n");
+        status = 1;
+        goto err_return;
+    }
+    if (lparen_count != rparen_count) {
+        fprintf(stderr, "ERROR (8) mismatched parentheses\n");
+        status = 1;
+        goto err_return;
+    }
     while (stack.top != -1) {
         ds_cat_printf(postfix_p, " %s", pop(&stack, &status));
         if (status) goto err_return;