diff --git a/example/math_expression/stack.go b/example/math_expression/stack.go
index b6c37b9..eebd476 100644
--- a/example/math_expression/stack.go
+++ b/example/math_expression/stack.go
@@ -1,3 +1,4 @@
+//go:build example
 // +build example
 
 package main
@@ -10,7 +11,7 @@ type lexemStack []unilex.Lexem
 
 func (ls *lexemStack) Head() (l unilex.Lexem) {
 	if len(*ls) == 0 {
-		return unilex.Lexem{Type: unilex.LEOF}
+		return unilex.Lexem{Type: unilex.LexEOF}
 	}
 	return (*ls)[len(*ls)-1]
 }
@@ -21,7 +22,7 @@ func (ls *lexemStack) Push(l unilex.Lexem) {
 
 func (ls *lexemStack) Pop() (l unilex.Lexem) {
 	if len(*ls) == 0 {
-		return unilex.Lexem{Type: unilex.LEOF}
+		return unilex.Lexem{Type: unilex.LexEOF}
 	}
 	*ls, l = (*ls)[:len(*ls)-1], (*ls)[len(*ls)-1]
 	return l
diff --git a/lexer.go b/lexer.go
index b556045..3b4da4d 100644
--- a/lexer.go
+++ b/lexer.go
@@ -134,16 +134,22 @@ func (l *Lexer) AcceptAnyOf(s []string, caseInsentive bool) bool {
 }
 
 // AcceptWhile passing symbols from input while they at `valid` string.
-func (l *Lexer) AcceptWhile(valid string) {
+func (l *Lexer) AcceptWhile(valid string) bool {
+	isValid := false
 	for l.Accept(valid) {
+		isValid = true
 	}
+	return isValid
 }
 
 // AcceptWhileNot passing symbols from input while they NOT in `invalid` string.
-func (l *Lexer) AcceptWhileNot(invalid string) {
+func (l *Lexer) AcceptWhileNot(invalid string) bool {
+	isValid := false
 	for !strings.ContainsRune(invalid, l.Next()) {
+		isValid = true
 	}
 	l.Back()
+	return isValid
 }
 
 // AtStart returns true if current lexem not empty