diff --git a/labs/04/lex_analaizer.l b/labs/04/lex_analaizer.l
new file mode 100644
index 00000000..913712e0
--- /dev/null
+++ b/labs/04/lex_analaizer.l
@@ -0,0 +1,43 @@
+%option noyywrap
+%{
+/*Performing lexical Analysis of a COMMENT, an INTEGER, and a FLOAT */
+
+#include <stdio.h> /*Para incluir la funcion de printf*/
+
+%}
+
+%%
+\/\/[^\/].*	{ printf("COMMENT\n"); }
+f	{ printf("floatdcl "); }
+i	{ printf("intdcl "); }
+=	{ printf("assign "); }
+\+	{ printf("plus "); }
+[0-9]+\.[0-9]+	{ printf("fnum\n"); }
+[0-9]+	{ printf("inum\n"); }
+[a-eg-hj-oq-z]	{ printf("id\n"); }
+p	{ printf("print "); }
+\/	{ printf("divide "); }	
+\*	{ printf("times "); }	
+\-	{ printf("minus "); }	
+
+[ \t\n]+	;
+%%
+
+int main(int argc, char **argv) {
+    FILE    *fd;
+
+    if (argc == 2)
+    {
+        if (!(fd = fopen(argv[1], "r")))
+        {
+            perror("Error: ");
+            return (-1);
+        }
+        yyset_in(fd);
+        yylex();
+        fclose(fd);
+    }
+    else
+        printf("Usage: a.out filename\n");
+    return (0);
+}