diff --git a/AlgebraicDataflowArchitectureModel/src/code/ast/Lexer.java b/AlgebraicDataflowArchitectureModel/src/code/ast/Lexer.java
new file mode 100644
index 0000000..cd2974c
--- /dev/null
+++ b/AlgebraicDataflowArchitectureModel/src/code/ast/Lexer.java
@@ -0,0 +1,55 @@
+package code.ast;
+
+/**
+ * The Lexer class provides functionalities for parsing and iterating through
+ * a source string of text character by character.
+ *
+ * @author s-yamagiwa;
+ */
+public class Lexer {
+	private final String source;
+	private int position;
+	
+	public Lexer(String source) {
+		this.source = source;
+		this.position = 0;
+	}
+	
+	/**
+	 * Retrieves the next character without advancing the position.
+	 *
+	 * @return The next character.
+	 */
+	public char peek() {
+		if (position >= source.length()) {
+			return '\0'; // End of line
+		}
+		
+		return source.charAt(position);
+	}
+	
+	/**
+	 * Retrieves the next character after advancing the position.
+	 *
+	 * @return The next character.
+	 */
+	public char peekNext() {
+		if (position + 1 >= source.length()) {
+			return '\0'; // End of line
+		}
+		
+		return source.charAt(position + 1);
+	}
+	
+	/**
+	 * Advances the position by one character.
+	 *
+	 * @return The character that was previously at the current position.
+	 */
+	public char advance() {
+		char current = peek();
+		position++;
+		
+		return current;
+	}
+}
diff --git a/AlgebraicDataflowArchitectureModel/src/code/ast/NumberLiteral.java b/AlgebraicDataflowArchitectureModel/src/code/ast/NumberLiteral.java
index 21ca1e3..c397e24 100644
--- a/AlgebraicDataflowArchitectureModel/src/code/ast/NumberLiteral.java
+++ b/AlgebraicDataflowArchitectureModel/src/code/ast/NumberLiteral.java
@@ -6,18 +6,54 @@
  * @author s-yamagiwa
  */
 public class NumberLiteral extends Expression {
+	/**
+	 * Represents the raw string value of a numeric token in the source code.
+	 * This variable stores the literal text that defines a numeric value, including
+	 * any prefixes, suffixes, or delimiters used in the representation of the number.
+	 */
 	private String tokenValue;
 	
+	/**
+	 * Constructs a new NumberLiteral instance with the specified token value.
+	 *
+	 * @param tokenValue - The literal text representing the numeric value in the source code.
+	 */
 	public NumberLiteral(String tokenValue) {
-		this.tokenValue = tokenValue;
+		setTokenValue(tokenValue);
 	}
 	
+	/**
+	 * Retrieves the raw string value of the numeric token in the source code.
+	 *
+	 * @return The raw string value of the numeric token.
+	 */
 	public String getTokenValue() {
 		return tokenValue;
 	}
 	
+	/**
+	 * Sets the raw string value of the numeric token in the source code.
+	 *
+	 * @param tokenValue - The raw string value of the numeric token.
+	 * @throws IllegalArgumentException if the specified token value is null, empty, or invalid number representation in Java source code.
+	 */
 	public void setTokenValue(String tokenValue) {
-		this.tokenValue = tokenValue;
+		if (tokenValue == null) {
+			throw new IllegalArgumentException("Token value must not be null.");
+		}
+		
+		int length = tokenValue.length();
+		if (length == 0) {
+			throw new IllegalArgumentException("Token value must not be empty.");
+		}
+		
+		// TODO: 0 check
+		// TODO: Binary number
+		// TODO: Octal number
+		
+		if (NumberUtil.isHexNumber(tokenValue)) {
+			this.tokenValue = tokenValue;
+		}
 	}
 	
 	@Override
diff --git a/AlgebraicDataflowArchitectureModel/src/code/ast/NumberUtil.java b/AlgebraicDataflowArchitectureModel/src/code/ast/NumberUtil.java
new file mode 100644
index 0000000..7d07105
--- /dev/null
+++ b/AlgebraicDataflowArchitectureModel/src/code/ast/NumberUtil.java
@@ -0,0 +1,64 @@
+package code.ast;
+
+/**
+ * Utility class that provides methods for number-related operations.
+ *
+ * @author s-yamagiwa
+ */
+public class NumberUtil {
+	/**
+	 * Determines whether the given string is a valid hexadecimal number.
+	 * A valid hexadecimal number starts with "0x" or "0X", followed by a
+	 * combination of digits (0-9), letters (a-f, A-F), or underscores ('_'),
+	 * with restrictions on the positioning of underscores.
+	 *
+	 * @param token The string to be evaluated as a hexadecimal number.
+	 * @return {@code true} if the given string represents a valid hexadecimal number,
+	 * {@code false} otherwise.
+	 * @apiNote This method returns {@code true} if the given token is a valid hexadecimal number representaion in <b>Java source code</b>.
+	 * So make sure that the specification of this method is determined by Java language specification.
+	 */
+	public static boolean isHexNumber(String token) {
+		if (token == null) {
+			return false;
+		}
+		
+		int length = token.length();
+		if (length < 3) {
+			return false;
+		}
+		
+		Lexer lexer = new Lexer(token.toLowerCase());
+		
+		if (lexer.peek() != '0' || lexer.peekNext() != 'x') {
+			return false;
+		}
+		
+		lexer.advance(); // 0
+		lexer.advance(); // x
+		
+		// Checks for cases like 0x or 0x_ (Invalid underscore position)
+		if (lexer.peek() == '\0' || lexer.peek() == '_') {
+			return false;
+		}
+		
+		char prev = lexer.peek();
+		while (lexer.peek() != '\0') {
+			char c = lexer.advance();
+			boolean isValidFormat = (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c == '_');
+			if (!isValidFormat) {
+				return false;
+			}
+			
+			// Save the previous character
+			prev = c;
+		}
+		
+		// Checks for cases like 0xFFFFF_ (Invalid underscore position)
+		if (prev == '_') {
+			return false;
+		}
+		
+		return true;
+	}
+}