[REGEX] :: POSIX ERE Meta Character '' 10. ? ( question mark ) ''

POSIX ERE( Extended Regular Expression )

9. +
10. ?
11. |

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10. ? ( Question Mark )

구문 ( Syntax )

<question_mark> ::= ?

• 사용 예

  <pattern_expr><question_mark>
  

설명 ( Description )

• 0 or 1
• 수량자(quantifier)로, <question_mark> 앞의 <pattern_expr>이 없거나 하나 있음을 의미한다.
  • ex) a? => 　, a
• <question_mark>만 있는 경우
  • ex) ?
  • 결과
    • Oracle : zero-length match
    • PostgreSQL, MySQL, MariaDB : error 처리

• zero-length match가 발생할 수 있다.

예제 ( Examples )

SQL> SELECT REGEXP_SUBSTR( 'god', 'goo?d' ) AS RESULT FROM DUAL;

RESULT
--------------------
god

SQL> SELECT REGEXP_SUBSTR( 'good', 'goo?d' ) AS RESULT FROM DUAL;

RESULT
--------------------
good

SQL> SELECT REGEXP_SUBSTR( 'goood', 'goo?d' ) AS RESULT FROM DUAL;

RESULT
--------------------
NULL

SQL> SELECT REGEXP_SUBSTR( 'ababc', '(ab)?' ) AS RESULT FROM DUAL;

RESULT
--------------------
ab

SQL> SELECT REGEXP_SUBSTR( 'knight', '.?night' ) AS RESULT FROM DUAL;

RESULT
--------------------
knight

SQL> SELECT REGEXP_SUBSTR( 'abc', 'z?' ) AS RESULT FROM DUAL;

RESULT
--------------------
NULL

SQL> SELECT REGEXP_COUNT( 'abc', 'z?' ) AS RESULT FROM DUAL;

    RESULT
----------
         4

• <question_mark>만 있는 경우

  < Oracle >
  SQL> SELECT REGEXP_SUBSTR( 'ab', '?' ) AS RESULT FROM DUAL;
  
  RESULT
  ------------------------------
  NULL
  
  SQL> SELECT REGEXP_COUNT( 'ab', '?' ) AS RESULT FROM DUAL;
  
      RESULT
  ----------
           3
  
  
  < PostgreSQL >
  postgres=# SELECT REGEXP_MATCHES( 'ab', '?', 'g' ) AS RESULT;
  ERROR:  invalid regular expression: quantifier operand invalid
  
  
  < MySQL >
  mysql> SELECT REGEXP_SUBSTR( 'ab', '?' ) AS RESULT;
  ERROR 3688 (HY000): Syntax error in regular expression on line 1, character 1.
  
  
  < MariaDB >
  MariaDB [test]> SELECT REGEXP_SUBSTR( 'ab', '?' ) AS RESULT;
  ERROR 1139 (42000): Got error 'nothing to repeat at offset 0' from regexp