Find the first word starting with each letter
Retina, 28 bytes:


M!i`\b(\w)(?&lt;!\b\1.+)\w*
¶
&#32



M! - Match each work and print all words separated by newlines.
i - Ignore case.
\b(\w) - Capture first letter of each word
(?&lt;!\b\1.+) - After matching the letter, check if there wasn't a previous word starting with the same letter. \1.+ ensures at least two characters, so we are skipping the current word.
\w* - match the rest of the word.
The above matches only words - all other characters are removed.
¶\n&#32; - Replace newlines with spaces.


Try it online!