textbox validation: only letters, no internal spaces

How do I validate what's typed in a textbox so that it contains only letters and no internal spaces?
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<input onkeyup="this.value=this.value.replace(/[^a-z]/ig, '')">

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weikelbobAuthor Commented:
OK works great, now how does it work?
It keeps replacing any character other than a thru z and A thru Z. /[^a-z]/ig is the regular expression to do so.
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This works good for me. How can you make the textbox allow text and numbers but not spaces?
Worked it out! :)

<input type="text" name="textbox" onkeyup="this.value=this.value.replace(/[^a-z,0-9]/ig, '')">
I want to give an alert box only when they type a letter in the textbox.
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