First of all, you'll need a table, which contains abbr. and its actual text message.
After that, you'll have to break the input into words, and replace the occurrences of abbr. with actual text message.
pseudo code
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getline(cin, userInput);
words = split_to_words(userInput);
for (constauto &word : words) {
if (found_in_abbr_table(word)) {
cout << actual_text(word)
}
else {
cout << word
}
}