If you want to modify case of captured groups, TextSoap uses $u, $U, $l, $L, $E instead of the backslash syntax of PCRE.
Note: This is not part of the ICU standard, as it does not support this kind of case transformation.
There is a bug that \U and \u are colorized because TextSoap used to support \uhhhh and \Uhhhhhhhh -- but that proved problematic.
The next update of TextSoap will no longer highlight those in blue.
For fun, I tweaked this a bit to be more general, so it will handle period and question marks, and any Unicode character considered lowercase.
In general, lookaheads and look behinds are a bit slower, so alternatively, you could do it like this:
Here, we just capture the end character, the space, and the lowercase start of the next sentence.
Then in the replacement, we use $U which will uppercase all letters (of which there is only the first letter of the sentence)