How to format an entire document using RegEx in Google Docs add-on. It will bold my name in string like "5argon : Hello World!"

GoogleDocsFormatBasic.gs

function formatInkVN() {
    var body = DocumentApp.getActiveDocument().getBody();

    //all text!
    var allText = body.editAsText();

    var regexString = "^.*[ ]:[ ]";
    var rangeElement = allText.findText(regexString);
    while (rangeElement != null) {
        var matchedText = rangeElement.getElement().asText();
        var begin = rangeElement.getStartOffset();
        var end = rangeElement.getEndOffsetInclusive();
        Logger.log(begin + " " + end);
        matchedText.setBold(false); //matchedText is the entire line, I want to reset the line first
        matchedText.setBold(begin, end, true); //Then I bold only the part that RegEx matches.

        rangeElement = allText.findText(regexString, rangeElement); //continue from previous point
    }
}