This type of replacement\substitution is not possible with native .NET and so it is not possible with Trados Studio at the moment.
Please see this thread for more details and suggestions:
https://community.rws.com/product-groups/trados-portfolio/trados-studio/f/studio/31610/regex-for-upper-case-in-first-word-of-segmentDepending on your source file type, you can also try to edit the files in Notepad++.