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A regular expression describes a set of strings. A regular expressions for the REGEXP operator may use any of the following special characters and constructs:

• . match any character (including carriage return and new line)

  SELECT function_name
  FROM `functions`
  WHERE function_name
  REGEXP 'anchor..'
  

• ^ match the beginning of a string

  SELECT function_name
  FROM `functions`
  WHERE function_name
  REGEXP '^a';
  

• $ match the end of a string

  SELECT function_name
  FROM `functions`
  WHERE function_description
  REGEXP 't$';
  

• [characters] match any characters or using range

  SELECT function_name
  FROM `functions`
  WHERE function_name
  REGEXP '^[a-d]';
  

Still using function table, let’s see the other developing using regex.
To view function_name which has 4 characters:

SELECT function_name
FROM `functions`
WHERE function_name
REGEXP '^....$';

Or we can write like this:

SELECT function_name
FROM `functions`
WHERE function_name
REGEXP '^.{4}$';

Hmm I think is enough for introduction of REGEXP .