Capitalization Converter

=TEXTJOIN(" ", TRUE, MAP(SPLIT(REGEXREPLACE(A1, "([.!?:])", "$1|"), "|"), LAMBDA(segment,

  LET(

    trimmed, TRIM(segment),

    IF(trimmed="", "",

      LET(

        words, SPLIT(trimmed, " "),

        punc, REGEXEXTRACT(trimmed, "[.!?:]$"),

        clean_words, IF(ISERROR(punc), words, SPLIT(SUBSTITUTE(trimmed, punc, ""), " ")),

        formatted, MAP(SEQUENCE(1, COUNTA(clean_words)), LAMBDA(i,

          LET(

            w, INDEX(clean_words, i),

            is_all_caps, EXACT(w, UPPER(w)),

            is_first, i=1,

            is_single_a, LOWER(w)="a",

            IF(is_all_caps, w,

              IF(is_first, REPLACE(LOWER(w), 1, 1, UPPER(LEFT(w, 1))),

                IF(is_single_a, "a", LOWER(w))

              )

            )

          )

        )),

        TEXTJOIN(" ", TRUE, formatted) & IF(ISERROR(punc), "", punc)

      )

    )

  )

)))