There is a bug with the pattern validator (tested IE 11.0.9600.16521)
The pattern attribute, per W3, should match the entire user input against the pattern, as if it had starting and ending anchors. There appears to be a bug where the regex engine does not check the entire string, leading to subtle problems with alterations, which may or may not match the entire string. Instead, it seems a second check is performed after the regex validator to check the length of the string is equal to the pattern match.
See stack overflow post http://stackoverflow.com/questions/22360235/do-browsers-support-different-html5-pattern-regexp-features
Per the W3 spec the pattern field should match