Fixed a fundamental bug where ^ would always match.

The engine was always passing a string sliced to the current position,
resulting in ^ always matching. Switched to use
FindRunesMatchStartingAt.

Fixes #242.

regexp_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/alecthomas/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestNewlineAtEndOfFile(t *testing.T) {
@@ -25,3 +26,21 @@ func TestNewlineAtEndOfFile(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, []Token{{Error, "hello"}}, it.Tokens())
 }
+
+func TestMatchingAtStart(t *testing.T) {
+	l := Coalesce(MustNewLexer(&Config{}, Rules{
+		"root": {
+			{`\s+`, Whitespace, nil},
+			{`^-`, Punctuation, Push("directive")},
+			{`->`, Operator, nil},
+		},
+		"directive": {
+			{"module", NameEntity, Pop(1)},
+		},
+	}))
+	it, err := l.Tokenise(nil, `-module ->`)
+	assert.NoError(t, err)
+	require.Equal(t,
+		[]Token{{Punctuation, "-"}, {NameEntity, "module"}, {Whitespace, " "}, {Operator, "->"}},
+		it.Tokens())
+}