Add a space after a ending dot of a sentence

For latest requirement:

rule = RegularExpression["(?!\\d\\.\\d)(\\w\\.+)(?! )(\\w?)"] -> "$1 $2";
StringReplace[rule] @ {text1, text2}

{"This is a sample text. Just 1.2 to test. To add a space after the dot... Okay. ", 
 "I watched football. 10 people played in 2 teams, my friend was player number 7. 20 minutes later the game ended with the score 2:1. Then I went home. "}

"Negative Lookahead" ("(?!...)") is used, BTW.

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Older-er-er response

StringReplace[text, RegularExpression["(\\w\\.)(\\w)"] -> "$1 $2"]

"\\w" means word characters, including letters, digits and the underscore _. "\\." means a period/dot literally. So the regular expression means to find a string pattern with length three: a word character followed by a period and followed by a word character.

Parentheses mean a group, and "$n" where n is an integer represents the contents in the n-th group.

So the whole operation is to add a blank between the two groups after locating them by the string pattern.

StringReplace[text, "." ~~a:Except[DigitCharacter|WhitespaceCharacter|"."] :> ". "<> a]

{"This is a sample text. Just 1.2 to test. To add a space after the dot... Okay."}

Also

StringReplace[text, a:LetterCharacter|"."~~"." ~~b:LetterCharacter:> a<>". "<>b ]

same result

StringReplace[StringReplace[text, "." -> ". "], ". " ~~ EndOfString -> "."]

{"This is a sample text. Just to test. To add a space after the dot. Okay."}