A Little Noise

Explode a string around a delimiter without a loop.

DROP PROCEDURE IF EXISTS explode;

DELIMITER |

CREATE PROCEDURE explode( pDelim VARCHAR(32), pStr TEXT)                                
BEGIN                                
   DROP TABLE IF EXISTS temp_explode;                                
   CREATE TEMPORARY TABLE temp_explode (id INT AUTO_INCREMENT PRIMARY KEY NOT NULL, word VARCHAR(40));                                
   SET @sql := CONCAT('INSERT INTO temp_explode (word) VALUES (', REPLACE(QUOTE(pStr), pDelim, '\'), (\''), ')');                                
   PREPARE myStmt FROM @sql;                                
   EXECUTE myStmt;                                
END |   

DELIMITER ;

-- example call:
SET @str  = "The quick brown fox jumped over the lazy dog"; 
SET @delim = " "; 

CALL explode(@delim,@str);
SELECT id,word FROM temp_explode;

Extract the first, longest pattern that matches a regular expression.
You could add a parameter or write a similar function to get the shortest match instead (by starting with e = s and then incrementing e, instead of starting with e = LENGTH(string) and decrementing).

CREATE FUNCTION REGEXP_EXTRACT(string TEXT, exp TEXT)
-- Extract the first longest string that matches the regular expression
-- If the string is 'ABCD', check all strings and see what matches: 'ABCD', 'ABC', 'AB', 'A', 'BCD', 'BC', 'B', 'CD', 'C', 'D'
-- It's not smart enough to handle things like (A)|(BCD) correctly in that it will return the whole string, not just the matching token.

RETURNS TEXT
DETERMINISTIC
BEGIN
  DECLARE s INT DEFAULT 1;
  DECLARE e INT;
  DECLARE adjustStart TINYINT DEFAULT 1;
  DECLARE adjustEnd TINYINT DEFAULT 1;

  -- Because REGEXP matches anywhere in the string, and we only want the part that matches, adjust the expression to add '^' and '$'
  -- Of course, if those are already there, don't add them, but change the method of extraction accordingly.

  IF LEFT(exp, 1) = '^' THEN 
    SET adjustStart = 0;
  ELSE
    SET exp = CONCAT('^', exp);
  END IF;

  IF RIGHT(exp, 1) = '$' THEN
    SET adjustEnd = 0;
  ELSE
    SET exp = CONCAT(exp, '$');
  END IF;

  -- Loop through the string, moving the end pointer back towards the start pointer, then advance the start pointer and repeat
  -- Bail out of the loops early if the original expression started with '^' or ended with '$', since that means the pointers can't move
  WHILE (s <= LENGTH(string)) DO
    SET e = LENGTH(string);
    WHILE (e >= 1) DO
      IF SUBSTRING(string, s, e) REGEXP exp THEN
        RETURN SUBSTRING(string, s, e);
      END IF;
      IF adjustEnd THEN
        SET e = e - 1;
      ELSE
        SET e = s - 1; -- ugh, such a hack to end it early
      END IF;
    END WHILE;
    IF adjustStart THEN
      SET s = s + 1;
    ELSE
      SET s = LENGTH(string) + 1; -- ugh, such a hack to end it early
    END IF;
  END WHILE;

  RETURN NULL;

END