Regex lookahead for 'not followed by' in grep

The answer to part of your problem is here, and ack would behave the same way: Ack & negative lookahead giving errors

You are using double-quotes for grep, which permits bash to "interpret ! as history expand command."

You need to wrap your pattern in SINGLE-QUOTES: grep 'Ui\.(?!L)' *

However, see @JonathanLeffler's answer to address the issues with negative lookaheads in standard grep!

You probably cant perform standard negative lookaheads using grep, but usually you should be able to get equivalent behaviour using the "inverse" switch '-v'. Using that you can construct a regex for the complement of what you want to match and then pipe it through 2 greps.

For the regex in question you might do something like

grep 'Ui\.' * | grep -v 'Ui\.L'

Negative lookahead, which is what you're after, requires a more powerful tool than the standard grep. You need a PCRE-enabled grep.

If you have GNU grep, the current version supports options -P or --perl-regexp and you can then use the regex you wanted.

If you don't have (a sufficiently recent version of) GNU grep, then consider getting ack.