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80 lines
3.2 KiB
Markdown
80 lines
3.2 KiB
Markdown
# Functions for searching and replacing in strings
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## replaceOne(haystack, 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)
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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)
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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)
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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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