Functions for searching and replacing in strings
------------------------------------------------

replaceOne(haystack, pattern, replacement)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Replaces the first occurrence, if it exists, of the 'pattern' substring in 'haystack' with the 'replacement' substring.
Hereafter, 'pattern' and 'replacement' must be constants.

replaceAll(haystack, pattern, replacement)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Replaces all occurrences of the 'pattern' substring in 'haystack' with the 'replacement' substring.

replaceRegexpOne(haystack, pattern, replacement)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Replacement using the 'pattern' regular expression. A re2 regular expression. Replaces only the first occurrence, if it exists.
A pattern can be specified as 'replacement'. This pattern can include substitutions \0-\9\.
The substitution \0 includes the entire regular expression.
The substitutions \1-\9 include the subpattern corresponding to the number.
In order to specify the \ symbol in a pattern, you must use a \ symbol to escape it.
Also keep in mind that a string literal requires an extra escape.

Example 1. Converting the date to American format:

.. code-block:: sql

  SELECT DISTINCT
      EventDate,
      replaceRegexpOne(toString(EventDate), '(\\d{4})-(\\d{2})-(\\d{2})', '\\2/\\3/\\1') AS res
  FROM test.hits
  LIMIT 7
  FORMAT TabSeparated
  
  2014-03-17      03/17/2014
  2014-03-18      03/18/2014
  2014-03-19      03/19/2014
  2014-03-20      03/20/2014
  2014-03-21      03/21/2014
  2014-03-22      03/22/2014
  2014-03-23      03/23/2014

Example 2. Copy the string ten times:

.. code-block:: sql

  SELECT replaceRegexpOne('Hello, World!', '.*', '\\0\\0\\0\\0\\0\\0\\0\\0\\0\\0') AS res
  
  ┌─res────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
  │ Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World! │
  └────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

replaceRegexpAll(haystack, pattern, replacement)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This does the same thing, but replaces all the occurrences. Example:

.. code-block:: sql
  SELECT replaceRegexpAll('Hello, World!', '.', '\\0\\0') AS res
  
  ┌─res────────────────────────┐
  │ HHeelllloo,,  WWoorrlldd!! │
  └────────────────────────────┘

As an exception, if a regular expression worked on an empty substring, the replacement is not made more than once. 
Example:

.. code-block:: sql
  SELECT replaceRegexpAll('Hello, World!', '^', 'here: ') AS res
  
  ┌─res─────────────────┐
  │ here: Hello, World! │
  └─────────────────────┘