diff --git a/file/error.go b/file/error.go
index 8ff85dfa5..b7838d4e1 100644
--- a/file/error.go
+++ b/file/error.go
@@ -3,7 +3,6 @@ package file
 import (
 	"fmt"
 	"strings"
-	"unicode/utf8"
 )
 
 type Error struct {
@@ -19,43 +18,49 @@ func (e *Error) Error() string {
 	return e.format()
 }
 
+var tabReplacer = strings.NewReplacer("\t", " ")
+
 func (e *Error) Bind(source Source) *Error {
+	src := source.String()
+
 	e.Line = 1
-	for i, r := range source {
-		if i == e.From {
+	var runeCount, lineStart, lineOffset int
+	for i, r := range src {
+		if runeCount == e.From {
 			break
 		}
 		if r == '\n' {
+			lineStart = i
 			e.Line++
 			e.Column = 0
+			lineOffset = 0
 		} else {
 			e.Column++
 		}
+		runeCount++
+		lineOffset++
+	}
+
+	lineEnd := lineStart + strings.IndexByte(src[lineStart:], '\n')
+	if lineEnd < lineStart {
+		lineEnd = len(src)
+	}
+	if lineStart == lineEnd {
+		return e
 	}
-	if snippet, found := source.Snippet(e.Line); found {
-		snippet := strings.Replace(snippet, "\t", " ", -1)
-		srcLine := "\n | " + snippet
-		var bytes = []byte(snippet)
-		var indLine = "\n | "
-		for i := 0; i < e.Column && len(bytes) > 0; i++ {
-			_, sz := utf8.DecodeRune(bytes)
-			bytes = bytes[sz:]
-			if sz > 1 {
-				goto noind
-			} else {
-				indLine += "."
-			}
-		}
-		if _, sz := utf8.DecodeRune(bytes); sz > 1 {
-			goto noind
-		} else {
-			indLine += "^"
-		}
-		srcLine += indLine
 
-	noind:
-		e.Snippet = srcLine
+	const prefix = "\n | "
+	line := src[lineStart:lineEnd]
+	snippet := new(strings.Builder)
+	snippet.Grow(2*len(prefix) + len(line) + lineOffset + 1)
+	snippet.WriteString(prefix)
+	tabReplacer.WriteString(snippet, line)
+	snippet.WriteString(prefix)
+	for i := 0; i < lineOffset; i++ {
+		snippet.WriteByte('.')
 	}
+	snippet.WriteByte('^')
+	e.Snippet = snippet.String()
 	return e
 }
 
diff --git a/file/source.go b/file/source.go
index 8e2b2d154..b11bb5f9d 100644
--- a/file/source.go
+++ b/file/source.go
@@ -1,48 +1,36 @@
 package file
 
-import (
-	"strings"
-	"unicode/utf8"
-)
+import "strings"
 
-type Source []rune
+type Source struct {
+	raw string
+}
 
 func NewSource(contents string) Source {
-	return []rune(contents)
+	return Source{
+		raw: contents,
+	}
 }
 
 func (s Source) String() string {
-	return string(s)
+	return s.raw
 }
 
 func (s Source) Snippet(line int) (string, bool) {
-	if s == nil {
+	if s.raw == "" {
 		return "", false
 	}
-	lines := strings.Split(string(s), "\n")
-	lineOffsets := make([]int, len(lines))
-	var offset int
-	for i, line := range lines {
-		offset = offset + utf8.RuneCountInString(line) + 1
-		lineOffsets[i] = offset
-	}
-	charStart, found := getLineOffset(lineOffsets, line)
-	if !found || len(s) == 0 {
-		return "", false
+	var start int
+	for i := 1; i < line; i++ {
+		pos := strings.IndexByte(s.raw[start:], '\n')
+		if pos < 0 {
+			return "", false
+		}
+		start += pos + 1
 	}
-	charEnd, found := getLineOffset(lineOffsets, line+1)
-	if found {
-		return string(s[charStart : charEnd-1]), true
-	}
-	return string(s[charStart:]), true
-}
-
-func getLineOffset(lineOffsets []int, line int) (int, bool) {
-	if line == 1 {
-		return 0, true
-	} else if line > 1 && line <= len(lineOffsets) {
-		offset := lineOffsets[line-2]
-		return offset, true
+	end := start + strings.IndexByte(s.raw[start:], '\n')
+	if end < start {
+		end = len(s.raw)
 	}
-	return -1, false
+	return s.raw[start:end], true
 }
diff --git a/parser/lexer/bench_test.go b/parser/lexer/bench_test.go
new file mode 100644
index 000000000..ccd6f6914
--- /dev/null
+++ b/parser/lexer/bench_test.go
@@ -0,0 +1,23 @@
+package lexer
+
+import (
+	"testing"
+
+	"github.com/expr-lang/expr/file"
+)
+
+func BenchmarkParser(b *testing.B) {
+	const source = `
+		/*
+			Showing worst case scenario
+		*/
+		let value = trim("contains escapes \n\"\\ \U0001F600 and non ASCII ñ"); // inline comment
+		len(value) == 0x2A
+		// let's introduce an error too
+		whatever
+	`
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		Lex(file.NewSource(source))
+	}
+}
diff --git a/parser/lexer/lexer.go b/parser/lexer/lexer.go
index e6b06c09d..8f067525f 100644
--- a/parser/lexer/lexer.go
+++ b/parser/lexer/lexer.go
@@ -3,18 +3,19 @@ package lexer
 import (
 	"fmt"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/expr-lang/expr/file"
 )
 
+const minTokens = 10
+
 func Lex(source file.Source) ([]Token, error) {
+	raw := source.String()
 	l := &lexer{
-		source: source,
-		tokens: make([]Token, 0),
-		start:  0,
-		end:    0,
+		raw:    raw,
+		tokens: make([]Token, 0, minTokens),
 	}
-	l.commit()
 
 	for state := root; state != nil; {
 		state = state(l)
@@ -28,10 +29,15 @@ func Lex(source file.Source) ([]Token, error) {
 }
 
 type lexer struct {
-	source     file.Source
+	raw        string
 	tokens     []Token
-	start, end int
 	err        *file.Error
+	start, end pos
+	eof        bool
+}
+
+type pos struct {
+	byte, rune int
 }
 
 const eof rune = -1
@@ -41,23 +47,39 @@ func (l *lexer) commit() {
 }
 
 func (l *lexer) next() rune {
-	if l.end >= len(l.source) {
-		l.end++
+	if l.end.byte >= len(l.raw) {
+		l.eof = true
 		return eof
 	}
-	r := l.source[l.end]
-	l.end++
+	r, sz := utf8.DecodeRuneInString(l.raw[l.end.byte:])
+	l.end.rune++
+	l.end.byte += sz
 	return r
 }
 
 func (l *lexer) peek() rune {
-	r := l.next()
-	l.backup()
-	return r
+	if l.end.byte < len(l.raw) {
+		r, _ := utf8.DecodeRuneInString(l.raw[l.end.byte:])
+		return r
+	}
+	return eof
+}
+
+func (l *lexer) peekByte() (byte, bool) {
+	if l.end.byte >= 0 && l.end.byte < len(l.raw) {
+		return l.raw[l.end.byte], true
+	}
+	return 0, false
 }
 
 func (l *lexer) backup() {
-	l.end--
+	if l.eof {
+		l.eof = false
+	} else if l.end.rune > 0 {
+		_, sz := utf8.DecodeLastRuneInString(l.raw[:l.end.byte])
+		l.end.byte -= sz
+		l.end.rune--
+	}
 }
 
 func (l *lexer) emit(t Kind) {
@@ -66,7 +88,7 @@ func (l *lexer) emit(t Kind) {
 
 func (l *lexer) emitValue(t Kind, value string) {
 	l.tokens = append(l.tokens, Token{
-		Location: file.Location{From: l.start, To: l.end},
+		Location: file.Location{From: l.start.rune, To: l.end.rune},
 		Kind:     t,
 		Value:    value,
 	})
@@ -74,11 +96,11 @@ func (l *lexer) emitValue(t Kind, value string) {
 }
 
 func (l *lexer) emitEOF() {
-	from := l.end - 2
+	from := l.end.rune - 1
 	if from < 0 {
 		from = 0
 	}
-	to := l.end - 1
+	to := l.end.rune - 0
 	if to < 0 {
 		to = 0
 	}
@@ -94,60 +116,37 @@ func (l *lexer) skip() {
 }
 
 func (l *lexer) word() string {
-	// TODO: boundary check is NOT needed here, but for some reason CI fuzz tests are failing.
-	if l.start > len(l.source) || l.end > len(l.source) {
-		return "__invalid__"
-	}
-	return string(l.source[l.start:l.end])
+	return l.raw[l.start.byte:l.end.byte]
 }
 
 func (l *lexer) accept(valid string) bool {
-	if strings.ContainsRune(valid, l.next()) {
+	if strings.ContainsRune(valid, l.peek()) {
+		l.next()
 		return true
 	}
-	l.backup()
 	return false
 }
 
 func (l *lexer) acceptRun(valid string) {
-	for strings.ContainsRune(valid, l.next()) {
+	for l.accept(valid) {
 	}
-	l.backup()
 }
 
 func (l *lexer) skipSpaces() {
-	r := l.peek()
-	for ; r == ' '; r = l.peek() {
-		l.next()
-	}
+	l.acceptRun(" ")
 	l.skip()
 }
 
-func (l *lexer) acceptWord(word string) bool {
-	pos := l.end
-
-	l.skipSpaces()
-
-	for _, ch := range word {
-		if l.next() != ch {
-			l.end = pos
-			return false
-		}
-	}
-	if r := l.peek(); r != ' ' && r != eof {
-		l.end = pos
-		return false
-	}
-
-	return true
-}
-
 func (l *lexer) error(format string, args ...any) stateFn {
 	if l.err == nil { // show first error
+		end := l.end.rune
+		if l.eof {
+			end++
+		}
 		l.err = &file.Error{
 			Location: file.Location{
-				From: l.end - 1,
-				To:   l.end,
+				From: end - 1,
+				To:   end,
 			},
 			Message: fmt.Sprintf(format, args...),
 		}
@@ -225,6 +224,6 @@ func (l *lexer) scanRawString(quote rune) (n int) {
 		ch = l.next()
 		n++
 	}
-	l.emitValue(String, string(l.source[l.start+1:l.end-1]))
+	l.emitValue(String, l.raw[l.start.byte+1:l.end.byte-1])
 	return
 }
diff --git a/parser/lexer/lexer_test.go b/parser/lexer/lexer_test.go
index db02d2acf..5171f4255 100644
--- a/parser/lexer/lexer_test.go
+++ b/parser/lexer/lexer_test.go
@@ -335,6 +335,7 @@ literal not terminated (1:10)
 früh ♥︎
 unrecognized character: U+2665 '♥' (1:6)
  | früh ♥︎
+ | .....^
 `
 
 func TestLex_error(t *testing.T) {
diff --git a/parser/lexer/token.go b/parser/lexer/token.go
index 459fa6905..c809c690e 100644
--- a/parser/lexer/token.go
+++ b/parser/lexer/token.go
@@ -31,17 +31,13 @@ func (t Token) String() string {
 }
 
 func (t Token) Is(kind Kind, values ...string) bool {
-	if len(values) == 0 {
-		return kind == t.Kind
+	if kind != t.Kind {
+		return false
 	}
-
 	for _, v := range values {
 		if v == t.Value {
-			goto found
+			return true
 		}
 	}
-	return false
-
-found:
-	return kind == t.Kind
+	return len(values) == 0
 }
diff --git a/parser/lexer/utils.go b/parser/lexer/utils.go
index 5c9e6b59d..fdb8beaa1 100644
--- a/parser/lexer/utils.go
+++ b/parser/lexer/utils.go
@@ -36,7 +36,8 @@ func unescape(value string) (string, error) {
 	if size >= math.MaxInt {
 		return "", fmt.Errorf("too large string")
 	}
-	buf := make([]byte, 0, size)
+	buf := new(strings.Builder)
+	buf.Grow(int(size))
 	for len(value) > 0 {
 		c, multibyte, rest, err := unescapeChar(value)
 		if err != nil {
@@ -44,13 +45,13 @@ func unescape(value string) (string, error) {
 		}
 		value = rest
 		if c < utf8.RuneSelf || !multibyte {
-			buf = append(buf, byte(c))
+			buf.WriteByte(byte(c))
 		} else {
 			n := utf8.EncodeRune(runeTmp[:], c)
-			buf = append(buf, runeTmp[:n]...)
+			buf.Write(runeTmp[:n])
 		}
 	}
-	return string(buf), nil
+	return buf.String(), nil
 }
 
 // unescapeChar takes a string input and returns the following info:
diff --git a/vm/vm_test.go b/vm/vm_test.go
index 91752a419..817fc6cc2 100644
--- a/vm/vm_test.go
+++ b/vm/vm_test.go
@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/expr-lang/expr/file"
 	"github.com/expr-lang/expr/internal/testify/require"
 
 	"github.com/expr-lang/expr"
@@ -609,10 +610,10 @@ func TestVM_DirectCallOpcodes(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			program := vm.NewProgram(
-				nil, // source
-				nil, // node
-				nil, // locations
-				0,   // variables
+				file.Source{}, // source
+				nil,           // node
+				nil,           // locations
+				0,             // variables
 				tt.consts,
 				tt.bytecode,
 				tt.args,
@@ -735,10 +736,10 @@ func TestVM_IndexAndCountOperations(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			program := vm.NewProgram(
-				nil, // source
-				nil, // node
-				nil, // locations
-				0,   // variables
+				file.Source{}, // source
+				nil,           // node
+				nil,           // locations
+				0,             // variables
 				tt.consts,
 				tt.bytecode,
 				tt.args,
@@ -1176,10 +1177,10 @@ func TestVM_DirectBasicOpcodes(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			program := vm.NewProgram(
-				nil, // source
-				nil, // node
-				nil, // locations
-				0,   // variables
+				file.Source{}, // source
+				nil,           // node
+				nil,           // locations
+				0,             // variables
 				tt.consts,
 				tt.bytecode,
 				tt.args,