Support omission of string slice low/high indices, BIT-1097.

Omission of the low index defaults to 0:

    s = "12345"; s[:3] == "123"

Omission of the high index defaults to length of the string:

    s = "12345"; s[3:] == "45"

src/parse.y

@@ -55,7 +55,7 @@
 %type <expr> opt_init
 %type <val> TOK_CONSTANT
 %type <re> pattern
-%type <expr> expr init anonymous_function
+%type <expr> expr opt_expr init anonymous_function
 %type <event_expr> event
 %type <stmt> stmt stmt_list func_body for_head
 %type <type> type opt_type enum_body
@@ -261,6 +261,13 @@ decl_list:
 	|
 	;
 
+opt_expr:
+		expr
+			{ $$ = $1; }
+	|
+			{ $$ = 0; }
+	;
+
 expr:
 		'(' expr ')'
 			{
@@ -418,11 +425,13 @@ expr:
 			$$ = new IndexExpr($1, $3);
 			}
 
-	|	expr '[' expr ':' expr ']'
+	|	expr '[' opt_expr ':' opt_expr ']'
 			{
 			set_location(@1, @6);
-			ListExpr* le = new ListExpr($3);
+			Expr* low = $3 ? $3 : new ConstExpr(new Val(0, TYPE_COUNT));
-			le->Append($5);
+			Expr* high = $5 ? $5 : new SizeExpr($1);
+			ListExpr* le = new ListExpr(low);
+			le->Append(high);
 			$$ = new IndexExpr($1, le, true);
 			}
 

testing/btest/Baseline/language.string-indexing/out

@@ -28,6 +28,41 @@ word[5] =
 word[6] = 
 word[7] = 
 word[100] = 
+word[:-100] = 
+word[:-7] = 
+word[:-6] = 
+word[:-5] = 
+word[:-4] = H
+word[:-3] = He
+word[:-2] = Hel
+word[:-1] = Help
+word[:0] = 
+word[:1] = H
+word[:2] = He
+word[:3] = Hel
+word[:4] = Help
+word[:5] = HelpA
+word[:6] = HelpA
+word[:7] = HelpA
+word[:100] = HelpA
+word[-100:] = HelpA
+word[-7:] = HelpA
+word[-6:] = HelpA
+word[-5:] = HelpA
+word[-4:] = elpA
+word[-3:] = lpA
+word[-2:] = pA
+word[-1:] = A
+word[0:] = HelpA
+word[1:] = elpA
+word[2:] = lpA
+word[3:] = pA
+word[4:] = A
+word[5:] = 
+word[6:] = 
+word[7:] = 
+word[100:] = 
+HelpA
 
 A
 1234

testing/btest/language/string-indexing.bro

@@ -24,6 +24,14 @@ s = "012345";
 for ( i in indices )
 	print fmt("word[%s] = %s", indices[i], word[indices[i]]);
 
+for ( i in indices )
+	print fmt("word[:%s] = %s", indices[i], word[:indices[i]]);
+
+for ( i in indices )
+	print fmt("word[%s:] = %s", indices[i], word[indices[i]:]);
+
+print word[:];
+
 print "";
 
 print "A";