Support single-quoted strings

It's trivial, and makes command lines much simpler on Windows where
the quote character is " (double quote). Thanks @jftuga for the
suggestion. Fixes #5.

interp/interp_test.go

@@ -121,6 +121,8 @@ BEGIN {
 		// TODO: tests for numeric strings from $ fields
 
 		// Other expressions: TODO ?: num str regex
+		{`BEGIN { print '\"' 'x' "y" '\"' }`, "", "\"xy\"\n", "", "syntax error"},
+
 		// Built-in variables: TODO
 		// Field expressions and assignment: TODO
 		// Assignment expressions: TODO
@@ -261,7 +263,6 @@ BEGIN { early() }
 		// {`BEGIN { printf "x" };; BEGIN { printf "y" }`, "", "xy", "", ""},
 
 		// Ensure syntax errors result in errors
-		{`BEGIN { }'`, "", "", `parse error at 1:10: unexpected '\''`, "syntax error"},
 		// {`{ $1 = substr($1, 1, 3) print $1 }`, "", "", "ERROR", "syntax error"},
 	}
 	for _, test := range tests {

lexer/lexer.go

@@ -248,9 +248,12 @@ func (l *Lexer) scan() (Position, Token, string) {
 		}
 		tok = NUMBER
 		val = string(runes)
-	case '"':
+	case '"', '\'':
+		// Note: POSIX awk spec doesn't allow single-quoted strings,
+		// but this helps without quoting, especially on Windows
+		// where the shell quote character is " (double quote).
 		runes := []rune{}
-		for l.ch != '"' {
+		for l.ch != ch {
 			c := l.ch
 			if c < 0 {
 				return l.pos, ILLEGAL, "didn't find end quote in string"

lexer/lexer_test.go

@@ -29,9 +29,13 @@ func TestLexer(t *testing.T) {
 
 		// String tokens
 		{`"foo"`, `1:1 string "foo"`},
-		{`"a\t\r\n\zb"`, `1:1 string "a\t\r\nzb"`},
+		{`"a\t\r\n\z\'\"b"`, `1:1 string "a\t\r\nz'\"b"`},
 		{`"x`, `1:3 <illegal> "didn't find end quote in string"`},
 		{"\"x\n\"", `1:3 <illegal> "can't have newline in string", 1:3 <newline> "", 2:2 <illegal> "didn't find end quote in string"`},
+		{`'foo'`, `1:1 string "foo"`},
+		{`'a\t\r\n\z\'\"b'`, `1:1 string "a\t\r\nz'\"b"`},
+		{`'x`, `1:3 <illegal> "didn't find end quote in string"`},
+		{"'x\n'", `1:3 <illegal> "can't have newline in string", 1:3 <newline> "", 2:2 <illegal> "didn't find end quote in string"`},
 
 		// Number tokens
 		{"0", `1:1 number "0"`},