select DOM elements by ID, tag name, or class name

annotated.js

// inspired by @techpriester: https://gist.github.com/988627

function(
  a,                         // take a simple selector like "name", "#name", or ".name", and
  b                          // an optional context, and
){
  a = a.match(/^(\W)?(.*)/); // split the selector into name and symbol.
  return(                    // return an element or list, from within the scope of
    b                        // the passed context
    || document              // or document,
  )[
    "getElement" + (         // obtained by the appropriate method calculated by
      a[1]
        ? a[1] == "#"      
          ? "ById"           // the node by ID,
          : "sByClassName"   // the nodes by class name, or
        : "sByTagName"       // the nodes by tag name,
    )
  ](
    a[2]                     // called with the name.
  )
}

index.js

function(a,b){a=a.match(/^(\W)?(.*)/);return(b||document)["getElement"+(a[1]?a[1]=="#"?"ById":"sByClassName":"sByTagName")](a[2])}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 Jed Schmidt <http://jed.is>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "cssSelect",

  "description": "select DOM elements by ID, tag name, or class name",

  "keywords": [
    "dom",
    "css",
    "id",
    "class",
    "tag"
  ]
}

I would advice you about that IE<9 doesn't support getElementsByClassName (source: caniuse.com) contrary to rudylattae comment.

You can save another byte by removing ^. Due to how the regular expressions is executed this does not make a difference in your case. And by the way, I tried to create getElementsByClassName in 140 bytes, which is missing in Internet Explorer <9.0, but currently it still is 162 bytes.