The following will do the job. It’s based on an answer I wrote to another question. Customize the transformTypedChar
function to suit your needs; my example capitalizes only the letters a-g.
If you need this on a textarea rather than an <input type="text">
then be aware that there are issues in IE <= 8 with line breaks that the following code doesn’t handle for the sake of brevity. You can find the cross browser function for obtaining the selection within a textarea here: Is there an Internet Explorer approved substitute for selectionStart and selectionEnd?
function transformTypedChar(charStr) {
return /[a-g]/.test(charStr) ? charStr.toUpperCase() : charStr;
}
document.getElementById("your_input_id").onkeypress = function(evt) {
var val = this.value;
evt = evt || window.event;
// Ensure we only handle printable keys, excluding enter and space
var charCode = typeof evt.which == "number" ? evt.which : evt.keyCode;
if (charCode && charCode > 32) {
var keyChar = String.fromCharCode(charCode);
// Transform typed character
var mappedChar = transformTypedChar(keyChar);
var start, end;
if (typeof this.selectionStart == "number" && typeof this.selectionEnd == "number") {
// Non-IE browsers and IE 9
start = this.selectionStart;
end = this.selectionEnd;
this.value = val.slice(0, start) + mappedChar + val.slice(end);
// Move the caret
this.selectionStart = this.selectionEnd = start + 1;
} else if (document.selection && document.selection.createRange) {
// For IE up to version 8
var selectionRange = document.selection.createRange();
var textInputRange = this.createTextRange();
var precedingRange = this.createTextRange();
var bookmark = selectionRange.getBookmark();
textInputRange.moveToBookmark(bookmark);
precedingRange.setEndPoint("EndToStart", textInputRange);
start = precedingRange.text.length;
end = start + selectionRange.text.length;
this.value = val.slice(0, start) + mappedChar + val.slice(end);
start++;
// Move the caret
textInputRange = this.createTextRange();
textInputRange.collapse(true);
textInputRange.move("character", start - (this.value.slice(0, start).split("\r\n").length - 1));
textInputRange.select();
}
return false;
}
};