'?' matches exactly zero or one repetition : Meta characters « Regular Expressions Functions « Oracle PL/SQL Tutorial

SQL>
SQL>
SQL> SELECT REGEXP_SUBSTR('Yababa dababa do','a.?a') FROM dual;

REG
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aba

SQL>

18.6.Meta characters
	18.6.1.	Meta characters
	18.6.2.	The period may be substituted for any letter and still maintain a match
	18.6.3.	Use the caret-anchor to insist the matching start at the beginning of the string
	18.6.4.	Asking for a match for a capital 'F' followed by any character
	18.6.5.	The search string cannot be anchored at the beginning and then searched from some other position
	18.6.6.	Negating Carets
	18.6.7.	A '^' followed by something else like an 'l' (a lowercase 'L')
	18.6.8.	Match a string where 'i' is followed by any one character and followed by another 'i'
	18.6.9.	Regexp_Substr
	18.6.10.	'*' matches zero or more repetitions
	18.6.11.	'+' matches one or more repetitions
	18.6.12.	'?' matches exactly zero or one repetition
	18.6.13.	Asking to match an 'a' and zero or more 'b's:
	18.6.14.	A series of 'b's immediately following the 'a'
	18.6.15.	Matching at least one 'b'
	18.6.16.	Want an 'e' followed by any number of other characters and then another 'e'
	18.6.17.	REGEXP_SUBSTR(value,'e.*e')
	18.6.18.	Non-greedy '?'
	18.6.19.	Empty Strings and the ? Repetition Character
	18.6.20.	The '?' says to match zero or one time