Full-Code.gs

// SET UP ON SHEETS

function onOpen(e) {
  var ui = SpreadsheetApp.getUi();

  ui.createMenu('Get Txns').addItem('Populate Sheets', 'processTxnEmails').addToUi();
}

// FILTERING EMAILS...

function getBankEmails() {
  // This function searches for messages from “GeNS” and with emails having subject containing
  // “GeNS transaction Alert …”
  var timeframe = 'newer_than:1h';
  var filter = timeframe + " AND subject:GeNS transaction Alert AND -label:scripting-emails";
  var threads = GmailApp.search(filter, 0, 10);
  var messages = [];

  for (var x in threads) {
    var thread = threads[x]
    messages.push(thread.getMessages()[0]);
  }
  return messages;
}

function displayEmails() {
  var templ = HtmlService.createTemplateFromFile('messages');
  templ.messages = getBankEmails();
  return templ.evaluate();
}

// Program Entry Point.

function doGet() {
  // return displayEmails();
  console.log('Starting script...');
  // return displayParsedData();
  return processTxnEmails();
}


// PARSING EMAIL MESSAGES...

function parseMessageBody(messages) {
  var txnData = [];

  for (var m = 0; m < messages.length; m++) {

    var text = messages[m].getPlainBody();
    Logger.log(m);

    // debit/credit amount and account balance.

    // var amountMatches = text.match(/NGN\s+(-?[\d\,\.]+)/g);

    var amountMatches = text.match(/Amount[\s\S]*?Value/);
    var balanceMatches = text.match(/Balance[\s\S]*?Available/);

    var txnTypeMatches = text.match(/(Debit|Credit)/);
    var txnDateMatches = text.match(/\d{2}-[A-z]{3}-\d{4}|\d{2}-?\d{2}-\d{4}/);
    var txnAccountMatches = text.match(/0147890929/);
    var txnDescMatches = text.match(/Description[\s\S]*?Amount/); // >>>>>V2.0

    Logger.log(txnDescMatches);
    // Logger.log('regex last index: ' + regDesc.lastIndex);


    var data = {};

    // get the first amount from the match, which is the debit/credit amount.
    data.txnAmount = amountMatches ? amountMatches[0] : "No Amount Found";
    Logger.log(data.txnAmount);

    data.balance = balanceMatches ? balanceMatches[0] : "No Balance Found";
    Logger.log(data.balance);

    data.txnType = txnTypeMatches ? txnTypeMatches[0]: "No Type Found";
    Logger.log(data.txnType);

    data.txnDesc = txnDescMatches ?  txnDescMatches[0] : "No Description Found";

    // CLEAN UP DESCRIPTION MATCHES
    data.txnDesc = data.txnDesc.replace(/(?: *[\n\r])+ */g, '');
    data.txnDesc = data.txnDesc.replace('Description', '');
    data.txnDesc = data.txnDesc.replace(' :', '');
    data.txnDesc = data.txnDesc.replace(':', '');
    data.txnDesc = data.txnDesc.replace('Amount', '');
    Logger.log(data.txnDesc);

    // CLEAN UP AMOUNT MATCHES
    data.txnAmount = data.txnAmount.replace(/(?: *[\n\r])+ */g, '');
    data.txnAmount = data.txnAmount.replace('Amount : ', '');
    data.txnAmount = data.txnAmount.replace('Value', '');

    // CLEAN UP BALAMCE MATCHES
    data.balance = data.balance.replace(/(?: *[\n\r])+ */g, '');
    data.balance = data.balance.replace('Balance:', '');
    data.balance = data.balance.replace('Available', '');


    data.txnDate = txnDateMatches ?  txnDateMatches[0] : "No Date Found";
    Logger.log(data.txnDate);

    data.txnAccount = txnAccountMatches ? txnAccountMatches[0] : "No Account Found";
    Logger.log(data.txnAccount);
    
    txnData.push(data);
  }
  Logger.log(txnData);
  return txnData;
}

function displayParsedData() {
  var templ = HtmlService.createTemplateFromFile('parsed');
  templ.txnData = parseMessageBody(getBankEmails());
  return templ.evaluate();
}

// SAVE DATA TO GOOGLE SHEETS

function saveToSheet(txnData){
  var expenseSpreadsheet = SpreadsheetApp.openByUrl("https://docs.google.com/spreadsheets/d/1VZBkiqoTmcnWcxvNCEgHwMLhy_sBof4Cbd0gmKvkxOk/edit#gid=0");

  var dataSheet = expenseSpreadsheet.getSheetByName("Data");
  Logger.log('Populating spreadsheet...');

  for (var txn = 0; txn < txnData.length; txn++) {

    dataSheet.appendRow([
      txnData[txn].txnDate,
      txnData[txn].txnAmount,
      txnData[txn].txnDesc,
      txnData[txn].txnType,
      txnData[txn].balance,
      txnData[txn].txnAccount
      ]
    );
  }
}

function processTxnEmails() {
  var messages = getBankEmails();
  var txnData = parseMessageBody(messages);
  saveToSheet(txnData);
  labelMessagesAsDone(messages);
  return true;
}

function labelMessagesAsDone(messages) {
  var labelName = 'scripting-emails';
  Logger.log('Processing labels...');
  var mylabel = GmailApp.getUserLabelByName(labelName);

  if (!mylabel) {
    mylabel = GmailApp.createLabel(labelName);
    Logger.log("Label doesn't exist, creating new " + labelName); 
  }

  for (var m = 0; m < messages.length; m++) {
    mylabel.addToThread(messages[m].getThread());
  }
}

messages.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <title>Message Display Test</title>
    <link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons1.css">
  </head>
  <body style="padding:3em;">
  <h1>Messages</h1>
    <ul>
    <? for(var m=0;m<messages.length;m++){ ?>
    <li><?= messages[m].getSubject() ?></li>
    <p><?= messages[m].getPlainBody() ?></p>
    <? } ?>
    </ul>
  </body>
</html>

parsed.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <title>Data parsed from emails</title>
    <link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons1.css">
  </head>
  <body style="padding:3em;">
    <h2>Parsed data from Messages</h2>
    <table style="width:800px">
    <thead>
    <tr>
    <th>Date</th><th>Amount</th><th>Balance</th><th>Type</th><th>Description</th><th>Acount Number</th>
    </tr>
    </thead>
    <tbody>
    <? for(var m=0;m<txnData.length;m++){ ?>
    <tr>
      <td><?= txnData[m].txnDate ?></td>
      <td><?= txnData[m].txnAmount ?></td>
      <td><?= txnData[m].balance ?></td>
      <td><?= txnData[m].txnType ?></td>
      <td><?= txnData[m].txnDesc ?></td>
      <td><?= txnData[m].txnAccount ?></td>
    </tr>
    <? }?>
    </tbody>
    </table>
    
  </body>
</html>

Added comments to view scripting progress.