Is there any way to see the string that was matched in grep?

**I'm not talking about -o option.** [Posix](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03) says: > The search for a matching sequence starts at the beginning of a string and stops when the **first sequence matching the expression is found**, where "first" is defined to mean "begins earliest in the string". If the pattern permits a variable number of matching characters and thus there is more than one such sequence starting at that point, the **longest such sequence is matched**. For example, the BRE "bb*" matches the second to fourth characters of the string "abbbc", and the ERE "(wee|week)(knights|night)" matches all ten characters of the string "weeknights". And I want to **verify** what is being said in posix and this tutorial [regTutorialSite](https://www.regular-expressions.info/posix.html) : > A POSIX-compliant engine will still find the **leftmost match**. If you **apply** Set|SetValue to Set or SetValue **once**, it **will match Set**. How to "apply once"? When i run **grep -o** the result is two strings: Set and SetValue, but not just " one leftmost " . That is, I read about one thing, but in practice I get something else. So, how to see what string was matched by regex? (Perhaps the question was formulated incorrectly or could have been better)