Mail Merge ID finding Task trigger

TaskMailMergeTrigger.trigger

trigger TaskMailMergeTrigger on Task (before insert) {
    
    //Assumes "Mail Merge ID: " *with the space* is always on the line before
    //the ID, which is all numbers
    Pattern mmID = Pattern.compile('Mail Merge ID: ([0-9]+)');
    
    for (Task t : trigger.new) {
        Matcher mTask = mmID.matcher(t.Description);
        if (mTask.find()) {
            //Assumes you have a text field, Mail_Merge_ID__c, on the Activity object 
            t.Mail_Merge_ID__c = mTask.group(1);
        }
    }
}

TestTaskMailMerge.class

@isTest
public class TestTaskMailMerge {
    
    @isTest
    public static void testWithMatch() {
        String mergeId = '50403161';
        Task hasMatch = new Task(OwnerId = UserInfo.getUserId(),
                                 Subject = 'Task with Mail Merge ID',
                                 Description = 'Mail Merge: Yes\n\nMail Merge ID: ' + mergeId + '\n\nMail Merge Name: Sample email',
                                 Status = 'Not Started',
                                 Priority = 'Normal'
        						);
        
        Test.startTest();
        insert hasMatch;
        Test.stopTest();
        
        hasMatch = [SELECT Id, Mail_Merge_ID__c FROM Task WHERE Id = :hasMatch.Id];
        System.assertEquals(mergeId, hasMatch.Mail_Merge_ID__c,'Merge Id field not correctly populated');
    }
    
    @isTest
    public static void testNoMatch() {
        Task noMatch = new Task(OwnerId = UserInfo.getUserId(),
                                 Subject = 'Plain old Task',
                                 Description = 'Just a regular Task description.',
                                 Status = 'Not Started',
                                 Priority = 'Normal'
        						);
        
        Test.startTest();
        insert noMatch;
        Test.stopTest();
        
        noMatch = [SELECT Id, Mail_Merge_ID__c FROM Task WHERE Id = :noMatch.Id];
        System.assert(String.isBlank(noMatch.Mail_Merge_ID__c),'Merge Id field is not blank');
    }
}