ClickHouse/docs/en/functions/string_replace_functions.md

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. Substitutions \1-\9 correspond to the subpattern numbers.To use the \ character in a template, escape it using \. Also keep in mind that a string literal requires an extra escape.

Example 1. Converting the date to American format:

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. Copying a string ten times:

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:

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:

SELECT replaceRegexpAll('Hello, World!', '^', 'here: ') AS res

┌─res─────────────────┐
│ here: Hello, World! │
└─────────────────────┘