package tars.model.task;

import tars.commons.exceptions.IllegalValueException;

/**
 * Represents a Task's priority in tars.
 */
public class Priority {
    public static final String MESSAGE_PRIORITY_CONSTRAINTS = "Task priority should be h / m / l";
    public static final String PRIORITY_VALIDATION_REGEX = "[\\p{Lower} ]+";

    public String priorityLevel;

    /**
     * Validates given task priority level.
     *
     * @throws IllegalValueException
     *             if given priority level string is invalid.
     */
    public Priority(String priorityLevel) throws IllegalValueException {
        assert priorityLevel != null;
        priorityLevel = priorityLevel.trim();
        if (!isValidPriorityLevel(priorityLevel)) {
            throw new IllegalValueException(MESSAGE_PRIORITY_CONSTRAINTS);
        }
        this.priorityLevel = priorityLevel;
    }

    /**
     * Returns true if a given string is a valid task priority level.
     */
    public static boolean isValidPriorityLevel(String level) {
        return level.matches(PRIORITY_VALIDATION_REGEX);
    }

    @Override
    public String toString() {
        return priorityLevel;
    }
    
    @Override
    public boolean equals(Object other) {
        return other == this // short circuit if same object
                || (other instanceof Priority // instanceof handles nulls
                && this.toString().equals(((Priority) other).toString())); // state check
    }

    public void setLevel(String priorityLevel) {
        this.priorityLevel = priorityLevel;
    }

}

package tars.model.task;

import tars.commons.exceptions.IllegalValueException;

/**
 * Represents a Task's priority in tars.
 */
public class Priority {
    public static final String MESSAGE_PRIORITY_CONSTRAINTS = "Task priority should be h / m / l";
    public static final String PRIORITY_VALIDATION_REGEX = "^[hml]$";

    public String priorityLevel;

    /**
     * Validates given task priority level.
     *
     * @throws IllegalValueException
     *             if given priority level string is invalid.
     */
    public Priority(String priorityLevel) throws IllegalValueException {
        assert priorityLevel != null;
        priorityLevel = priorityLevel.trim();
        if (!isValidPriorityLevel(priorityLevel)) {
            throw new IllegalValueException(MESSAGE_PRIORITY_CONSTRAINTS);
        }
        this.priorityLevel = priorityLevel;
    }

    /**
     * Returns true if a given string is a valid task priority level.
     */
    public static boolean isValidPriorityLevel(String level) {
        return level.equals("") ? true : level.matches(PRIORITY_VALIDATION_REGEX);
    }

    @Override
    public String toString() {
        return priorityLevel;
    }
    
    @Override
    public boolean equals(Object other) {
        return other == this // short circuit if same object
                || (other instanceof Priority // instanceof handles nulls
                && this.toString().equals(((Priority) other).toString())); // state check
    }

    public void setLevel(String priorityLevel) {
        this.priorityLevel = priorityLevel;
    }

}

Update Priority - Update regex validation - Update isValidPriorityLevel method