Question

In MySQL I need to remove all single characters from a string:

"A Quick Brown B C D Fox" -> "Quick Brown Fox"

Any tips ?

Answer 1

You want [[:<:]] and [[:>:]] to search for word boundries in MySQL. (It would be \b in other regular expressions, PERL, grep, etc.)

So…

'[[:<:]][a-zA-Z0-9][[:>:]]'

Would find any single character.

OK TRY THIS…

This is based upon the function above, but hardcoded to look for single characters. I don’t claim this to be much more than a hack. But if you need to get the job done, and you cant install the external libraries for whatever reason, it won’t slow you down on strings that do NOT have any single characters to replace:

delimiter //
DROP FUNCTION IF EXISTS `REMOVE_SINGLE_CHARS`//
CREATE FUNCTION `REMOVE_SINGLE_CHARS`(original varchar(1000)) RETURNS varchar(1000) CHARSET utf8 DETERMINISTIC
BEGIN

DECLARE rxChars VARCHAR(20);
DECLARE temp VARCHAR(1000);
DECLARE ch1 CHAR(1);
DECLARE ch2 CHAR(1);
DECLARE ch3 CHAR(1);
DECLARE i INT;

SET rxChars = '[a-zA-Z0-9]';
SET i = 1;
SET temp = "";

IF original REGEXP CONCAT('[[:<:]]',rxChars,'[[:>:]]') THEN
  loop_label: LOOP
    set ch1 = SUBSTRING(original,i,1);
    if CHAR_LENGTH(original) > i THEN 
      set ch2 = SUBSTRING(original,i+1,1);
    END IF;
    if CHAR_LENGTH(original) > i+1 THEN 
      set ch3 = SUBSTRING(original,i+2,1);
    END IF;
    if(i = 1) THEN
        IF (ch1 NOT regexp rxChars OR ch2 regexp rxChars) THEN
          set temp = CONCAT(temp, ch1);
        END IF;
    END IF;

    IF(ch2 = ' ') THEN
      # Theoretically this is redundant, but for some reason the
      # CONCAT(temp, ch2) below is not working when ch2 = ' '   YMMV
      set temp = CONCAT(temp, ' ');
    ELSE 
      IF(ch2 NOT regexp rxChars OR ch1 regexp rxChars OR (i+1<CHAR_LENGTH(original) AND ch3 regexp rxChars)) THEN
        set temp = CONCAT(temp, ch2);
      END IF;
    END IF;

    IF i+2>CHAR_LENGTH(original) THEN
      LEAVE loop_label;
    END IF;
    SET i=i+1;
  END LOOP;
ELSE
  SET temp = original;
END IF;
RETURN temp;

END//