How to quickly type underscore

I use a javascript bookmarklet for typing unicode symbols at math.stackexchange.com. Mathjax renders most unicode the same as the corresponding latex macros. For example $ℝ$ and $\mathbb{R}$ give the same result. I like the way tex code stays more compact and readable with unicode symbols.

I think this code is able to do what you want. I like to use not too many keystrokes, so instead of \alpha I use \a to produce α. You can modify this script to your own needs, and then convert it to a bookmarklet, using this website for example: http://jasonmillerdesign.com/Free_Stuff/Instant_Bookmarklet_Converter

If you want to use this script on a website without jquery, then you first need to run this bookmarklet: http://www.learningjquery.com/2006/12/jquerify-bookmarklet/

jQuery.fn.autocorrect = function(options)
{
    if ("text" != jQuery(this).attr("type") && !jQuery(this).is("textarea"))
    {
        return;
    }
var defaults = {
        corrections: {
            a: "α",
            b: "β",
            c: "γ",
            d: "δ",
            e: "ϵ",
            emp : "∅",
            f: "\\frac{}{}",
            in : "∈",
            s: "σ",
            t: "\\text{}",
            tau : "τ",
            th : "θ",
            p: "π",
            pm : "±",
            o : "ω",
            O : "Ω",
            r : "ρ",
            A : "∀",
            E : "∃",
            R: "ℝ",
            C: "ℂ",
            H: "ℍ",
            N: "ℕ",
            Q: "ℚ",
            Z: "ℤ",
            int: "\\int_{}^{}",
            inf : "∞",
            sum : "\\sum_{}^{}",
            "-1": "^{-1}",
            ph: "ϕ",
            ch: "χ",
            ps: "ψ",
            leq : "≥",
            xi : "ξ", 
            geq : "≤",
            "/=" : "≠",
            "==" : "≡",
            "<" : "\\langle {} \\rangle",
            "->" : "→",
            "=>" : "⇒",
            "<=" : "⇐",
            "<>" : "⇔",
            "sq" : "\\sqrt{}"
    }
};
if (options && options.corrections)
{
    options.corrections = jQuery.extend(defaults.corrections, options.corrections);
}
var opts = jQuery.extend(defaults, options);
getCaretPosition = function(oField)
{
    var iCaretPos = 0;
    if (document.selection)
    {
        var oSel = document.selection.createRange();
        oSel.moveStart("character", 0 - oField.value.length);
        iCaretPos = oSel.text.length;
    }
    else if (oField.selectionStart || oField.selectionStart == "0")
    {
        iCaretPos = oField.selectionStart;
    }
    return (iCaretPos);
}
function setCaretPosition (oField, iCaretPos)
{
    if (document.selection)
    {
        var oSel = document.selection.createRange();
        oSel.moveStart("character", 0 - oField.value.length);
        oSel.moveStart("character", iCaretPos);
        oSel.moveEnd("character", 0);
    }
    else if (oField.selectionStart || oField.selectionStart == "0")
    {
        oField.selectionStart = iCaretPos;
        oField.selectionEnd = iCaretPos;
    }
}
this.keyup(function(e)
{
    if (32 != e.keyCode)
    {
        return;
    }
    var caretPosition = (getCaretPosition(this) - 1);
    if (1 > caretPosition)
    {
        return;
    }
    var valueOfField = this.value;
    var stringUptoCaretPosition = (valueOfField).substr(0, caretPosition);
    if (" " == stringUptoCaretPosition.charAt(caretPosition - 1))
    {
        return;
    }
    var beginIndex = stringUptoCaretPosition.lastIndexOf('\\');
    if (beginIndex < stringUptoCaretPosition.lastIndexOf(' '))
    {
        return;
    }
    var stringToSearch = stringUptoCaretPosition.substring(beginIndex+1);
    var stringNotToSearch = stringUptoCaretPosition.substring(0, beginIndex);
    if (!opts.corrections[stringToSearch])
    {
        return;
    }
    var stringToReplace = opts.corrections[stringToSearch];
    stringUptoCaretPosition = stringNotToSearch+ stringToReplace;
    var stringFromCaretPositionUptoEnd = (valueOfField).substr(caretPosition+1);
    this.value = (stringUptoCaretPosition + stringFromCaretPositionUptoEnd);
    if (stringToReplace.indexOf("{}")!=-1 )
    {
    setCaretPosition(this, stringUptoCaretPosition.indexOf("{}")+1);
    }
    else { setCaretPosition(this, stringUptoCaretPosition.length);}

});
};
$(document).ready(function()
{
    $("textarea").autocorrect();
});

Solution with Autohotkey

To use Autohotkey is probably the fastest solution. It works globally in all Windows programs. You need this Autohotkey script: greekletters.ahk, it works like this:

After installing the script you simply activate ⇪ Capslock, then you type for instance the key a which turns immediately into α. As fast as it can be.

To get capitalized letters, activate ⇪ Capslock, then hold Shift and press the letter you need.

Solution with Google Docs

Máté Juhász gave a great answer, which works if you have installed Microsoft Word 2007 or higher.

For everybody who has not Microsoft Word available, you can use Google Docs as a workaround. There you specify the Automatic corrections, go to >Tools >Preferences then insert for instance:

The same autoreplacement I can do now with Notepad++ using the Fingertext plugin.

So "Autocorrect" (automatic replacements) is the way to go!

Thanks to all for your ideas.