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* Replace underscores with hyphens * remove temporary code * fix style check * fix collapse
93 lines
4.2 KiB
Markdown
93 lines
4.2 KiB
Markdown
---
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toc_priority: 42
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toc_title: For Replacing in Strings
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---
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# Functions for Searching and Replacing in Strings {#functions-for-searching-and-replacing-in-strings}
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## replaceOne(haystack, pattern, replacement) {#replaceonehaystack-pattern-replacement}
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Replaces the first occurrence, if it exists, of the ‘pattern’ substring in ‘haystack’ with the ‘replacement’ substring.
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Hereafter, ‘pattern’ and ‘replacement’ must be constants.
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## replaceAll(haystack, pattern, replacement), replace(haystack, pattern, replacement) {#replaceallhaystack-pattern-replacement-replacehaystack-pattern-replacement}
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Replaces all occurrences of the ‘pattern’ substring in ‘haystack’ with the ‘replacement’ substring.
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## replaceRegexpOne(haystack, pattern, replacement) {#replaceregexponehaystack-pattern-replacement}
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Replacement using the ‘pattern’ regular expression. A re2 regular expression.
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Replaces only the first occurrence, if it exists.
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A pattern can be specified as ‘replacement’. This pattern can include substitutions `\0-\9`.
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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 `\`.
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Also keep in mind that a string literal requires an extra escape.
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Example 1. Converting the date to American format:
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``` sql
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SELECT DISTINCT
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EventDate,
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replaceRegexpOne(toString(EventDate), '(\\d{4})-(\\d{2})-(\\d{2})', '\\2/\\3/\\1') AS res
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FROM test.hits
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LIMIT 7
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FORMAT TabSeparated
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```
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``` text
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2014-03-17 03/17/2014
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2014-03-18 03/18/2014
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2014-03-19 03/19/2014
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2014-03-20 03/20/2014
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2014-03-21 03/21/2014
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2014-03-22 03/22/2014
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2014-03-23 03/23/2014
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```
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Example 2. Copying a string ten times:
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``` sql
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SELECT replaceRegexpOne('Hello, World!', '.*', '\\0\\0\\0\\0\\0\\0\\0\\0\\0\\0') AS res
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```
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``` text
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┌─res────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
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│ Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World!Hello, World! │
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└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
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```
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## replaceRegexpAll(haystack, pattern, replacement) {#replaceregexpallhaystack-pattern-replacement}
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This does the same thing, but replaces all the occurrences. Example:
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``` sql
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SELECT replaceRegexpAll('Hello, World!', '.', '\\0\\0') AS res
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```
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``` text
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┌─res────────────────────────┐
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│ HHeelllloo,, WWoorrlldd!! │
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└────────────────────────────┘
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```
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As an exception, if a regular expression worked on an empty substring, the replacement is not made more than once.
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Example:
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``` sql
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SELECT replaceRegexpAll('Hello, World!', '^', 'here: ') AS res
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```
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``` text
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┌─res─────────────────┐
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│ here: Hello, World! │
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└─────────────────────┘
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```
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## regexpQuoteMeta(s) {#regexpquotemetas}
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The function adds a backslash before some predefined characters in the string.
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Predefined characters: ‘0’, ‘\\’, ‘\|’, ‘(’, ‘)’, ‘^’, ‘$’, ‘.’, ‘\[’, ’\]’, ‘?’, ’\*‘,’+‘,’{‘,’:‘,’-’.
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This implementation slightly differs from re2::RE2::QuoteMeta. It escapes zero byte as \\0 instead of 00 and it escapes only required characters.
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For more information, see the link: [RE2](https://github.com/google/re2/blob/master/re2/re2.cc#L473)
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[Original article](https://clickhouse.tech/docs/en/query_language/functions/string_replace_functions/) <!--hide-->
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