Ok, check the following screenshot of a Search and Replace window in Trados Studio 2017 SR1. This is something that should NOT happen, and yet it happens in many Trados Studio versions (I am now working with Trados Studio 2017 SR1 but I think this also happened in Studio 2017, Studio 2015, and probably 2014 as well).

If you see the image, you can see I am trying to replace “(c” with “(C”. that is, capitalizing the “c” and leaving an open parenthesis at the beginning. There should be absolutely no problem with the replacement, but the window shows the following expression.

"The regular expression has an error and cannot be used:" etc etc.

As you can also see, the Use Regular Expression checkbox is NOT checked, And yet the window won't let me either perform a Search or Replace operation.

I believe the problem comes from the fact that I accidentally checked the Use Regular Expressions after writing the Search expression and then unchecked it, but the system still believes the Use Regular Expressions checkbox IS ticked. 

Can you believe that the only way to solve this and search for “(c” now is closing and restarting Studio???? I have tried multiple workarounds, such as ticking and unticking, performing a Search of a normal text string and others.. and nothing, the Use regular expressions checkbox seems to be “stuck”.

The funny thing, though, is that even though it seems to be Stuck, if the checkbox is unchecked, it won't perform a regex search. For example, with the configuration I show in the screenshot, and not ticking anything else, I can search strings such as “judgment", but not “(IV)”. And if I perform a search such as “\s”. which, if Regex expressions were activated, would match any space in the text, it does not find anything because I don't have the literal string “\s” (with a backlash) in my text.

So the only solution is to restart Trados. As I have said, I have noticed this behaviour in many Trados versions, but currently I am working in Studio 2017 SR1. Feel free to ask any other questions!