Bob Phillips wrote...
Not a formula, but you can do it with event code
....

And nasty users can turn off event handlers either by turning off all
macros (setting security to High would be one way), or running a macro
containing Application.EnableEvents = False .

If only the first 45 chars at most should be used, then formulas
referencing the cell should do so as LEFT(cell,45). It's always safer
to assume that ALL error handling outside of formulas has been
compromised and design downstream formulas accordingly.