test.js

function setSentenceCase() {

	// get the start offset and the selected text
	TXTextControl.selection.getStart(function(start) {
		TXTextControl.selection.getText(function(curText) {
		
			// in Text Control, CRLF is one character
			text = curText.split("\r\n").join("\n");
		
			// get all matches including indexes
			var matchesAll = [...text.matchAll(/\.\ +/g)];
			
			// to get the actual length, another match is required in JS
			var matches = [...text.match(/\.\ +/g)];
		
			const indexes = matchesAll.map(match => match.index);
			
			// loop through array of indexes
			for (let index = 0; index < indexes.length; ++index) {
				const element = indexes[index];
				
				var sel = TXTextControl.selection;
				
				var bounds = { "start": matches[index].length + start + element, "length": 1 };
				sel.setBounds(bounds);
				sel.setText(text.charAt(matches[index].length + element).toUpperCase());
			}
			
		})

	})
}