It appears this is related to the IE 7 lookahead bug documented here http://blog.stevenlevithan.com/archives/regex-lookahead-bug Changing the above regex to this resolved the problem (replaced the + with {n,m})
// Due to lookahead bug in IE 7, used {n,m} instead of just +
$(".alpha").inputmask('Regex', { regex: "[a-zA-Z ]{1,50}" }); // Limits entry to letters and space character
$(".name").inputmask('Regex', { regex: "[a-zA-Z-`' ]{1,50}" }); // Limits entry to letters, -, `, ' and space character
$(".alphanumeric").inputmask('Regex', { regex: "[a-zA-Z0-9 ]{1,50}" }); // Limits entry to letters, numbers and space character