Try posting your attachment then we can understand the context a little better. To most queries involving shortcomings of Thunderbird's filters, I recommend FiltaQuilla or Expression Search, both of which add limited regular expression ("RE") capability to the filters. An RE will respect the space character. Expression Search also enhances the QuickFilters, so it is useful in both filters and searches. FiltaQuilla, as its name implies, is targeted more towards filters. /[ \n\r\t\xa0]my /i would identify the word ''my'', in any mix of upper and lowercase, preceded by any of space, newline, return, tab or non-breaking space. So it would reject ''academy''. "My" is a rather "small change" sort of word, frequently used. I'm surprised that it would be a useful search term. I wonder how many words appear where "my" appears as an embedded element appear? Or is this just a contrived example?