Google Apps Script to send different auto-replies based on keyword in email body

Explanation / Issues:

You have one obvious issue with your code and one additional modification you need to do according to your comments:

• Replace if(msg = "test") with if(msg == "test") otherwise the first if block statement will be always executed.

• According to your comments msg is not just a single word but a text. And you want to check if test is between two words. Assuming that these words are this and that you can use the following regular expression to see if test is between this and that:

  const regex = RegExp(/(?<=this.*?)test(?=.*?that)/);
  regex.test(msg) // this returns true if test is between this and that
  

The RegExp.prototype.test() method allows you to validate your regular expression and get true if there is a match between the regular expression and a specified string, in your case msg.

Solution:

function autoReplier() 
{var labelObj = GmailApp.getUserLabelByName('autoreply');
var gmailThreads;
var messages;
const regex = RegExp(/(?<=this.*?)test(?=.*?that)/); //modify this and that if you want to use other words
var sender;
for (var gg = 0; gg < labelObj.getUnreadCount(); gg++) {
gmailThreads = labelObj.getThreads()[gg];
messages = gmailThreads.getMessages();
for (var ii = 0; ii < messages.length; ii++) 
  {if (messages[ii].isUnread()){msg = messages[ii].getPlainBody();
    if (regex.test(msg)) {
        sender = messages[ii].getFrom();
        MailApp.sendEmail(sender, "Auto Reply", "This is my test");                                                                     
        messages[ii].markRead();
        messages[ii].moveToTrash();
    } else {
        sender = messages[ii].getFrom();
        MailApp.sendEmail(sender, "Auto Reply", "Sorry, your keyword was not recognized");                                                                         
        messages[ii].markRead();
        messages[ii].moveToTrash();
}}}}
}