GetElementByTagName

I am trying to write a code that replaces HTML in a Table Row. I am in a situation where I can't put an ID on the table because I don't have full control over the document. I am using a remotely hosted forum called proboards.com and it allows you to put code into the headers and footers of the forum. So there are multiple tables. Here is what I have so far:

document.getElementsByTagName('tr').innerHTML=document.getElementsbyTagName('tr').innerHTML(/Whatever/,'the')

What am I doing wrong?