ClickHouse/docs/en/query_language/functions/string_search_functions.md

Functions for Searching Strings

The search is case-sensitive by default in all these functions. There are separate variants for case insensitive search.

position(haystack, needle), locate(haystack, needle)

Search for the substring needle in the string haystack. Returns the position (in bytes) of the found substring, starting from 1, or returns 0 if the substring was not found.

For a case-insensitive search, use the function positionCaseInsensitive.

positionUTF8(haystack, needle)

The same as position, but the position is returned in Unicode code points. Works under the assumption that the string contains a set of bytes representing a UTF-8 encoded text. If this assumption is not met, it returns some result (it doesn't throw an exception).

For a case-insensitive search, use the function positionCaseInsensitiveUTF8.

multiSearchAllPositions(haystack, [needle₁, needle₂, ..., needle_n])

The same as position, but returns Array of the positions for all needle_i.

For a case-insensitive search or/and in UTF-8 format use functions multiSearchAllPositionsCaseInsensitive, multiSearchAllPositionsUTF8, multiSearchAllPositionsCaseInsensitiveUTF8.

multiSearchFirstPosition(haystack, [needle₁, needle₂, ..., needle_n])

The same as position but returns the leftmost offset of the string haystack that is matched to some of the needles.

For a case-insensitive search or/and in UTF-8 format use functions multiSearchFirstPositionCaseInsensitive, multiSearchFirstPositionUTF8, multiSearchFirstPositionCaseInsensitiveUTF8.

multiSearchFirstIndex(haystack, [needle₁, needle₂, ..., needle_n])

Returns the index i (starting from 1) of the leftmost found needle_i in the string haystack and 0 otherwise.

For a case-insensitive search or/and in UTF-8 format use functions multiSearchFirstIndexCaseInsensitive, multiSearchFirstIndexUTF8, multiSearchFirstIndexCaseInsensitiveUTF8.

multiSearchAny(haystack, [needle₁, needle₂, ..., needle_n])

Returns 1, if at least one string needle_i matches the string haystack and 0 otherwise.

For a case-insensitive search or/and in UTF-8 format use functions multiSearchAnyCaseInsensitive, multiSearchAnyUTF8, multiSearchAnyCaseInsensitiveUTF8.

!!! note "Note" In all multiSearch* functions the number of needles should be less than 2⁸ because of implementation specification.

match(haystack, pattern)

Checks whether the string matches the pattern regular expression. A re2 regular expression. The syntax of the re2 regular expressions is more limited than the syntax of the Perl regular expressions.

Returns 0 if it doesn't match, or 1 if it matches.

Note that the backslash symbol (\) is used for escaping in the regular expression. The same symbol is used for escaping in string literals. So in order to escape the symbol in a regular expression, you must write two backslashes (\) in a string literal.

The regular expression works with the string as if it is a set of bytes. The regular expression can't contain null bytes. For patterns to search for substrings in a string, it is better to use LIKE or 'position', since they work much faster.

multiMatchAny(haystack, [pattern₁, pattern₂, ..., pattern_n])

The same as match, but returns 0 if none of the regular expressions are matched and 1 if any of the patterns matches. It uses hyperscan library. For patterns to search substrings in a string, it is better to use multiSearchAny since it works much faster.

!!! note "Note" The length of any of the haystack string must be less than 2³² bytes otherwise the exception is thrown. This restriction takes place because of hyperscan API.

multiMatchAnyIndex(haystack, [pattern₁, pattern₂, ..., pattern_n])

The same as multiMatchAny, but returns any index that matches the haystack.

multiFuzzyMatchAny(haystack, distance, [pattern₁, pattern₂, ..., pattern_n])

The same as multiMatchAny, but returns 1 if any pattern matches the haystack within a constant edit distance. This function is also in an experimental mode and can be extremely slow. For more information see hyperscan documentation.

multiFuzzyMatchAnyIndex(haystack, distance, [pattern₁, pattern₂, ..., pattern_n])

The same as multiFuzzyMatchAny, but returns any index that matches the haystack within a constant edit distance.

!!! note "Note" multiFuzzyMatch* functions do not support UTF-8 regular expressions, and such expressions are treated as bytes because of hyperscan restriction.

!!! note "Note" To turn off all functions that use hyperscan, use setting SET allow_hyperscan = 0;.

extract(haystack, pattern)

Extracts a fragment of a string using a regular expression. If 'haystack' doesn't match the 'pattern' regex, an empty string is returned. If the regex doesn't contain subpatterns, it takes the fragment that matches the entire regex. Otherwise, it takes the fragment that matches the first subpattern.

extractAll(haystack, pattern)

Extracts all the fragments of a string using a regular expression. If 'haystack' doesn't match the 'pattern' regex, an empty string is returned. Returns an array of strings consisting of all matches to the regex. In general, the behavior is the same as the 'extract' function (it takes the first subpattern, or the entire expression if there isn't a subpattern).

like(haystack, pattern), haystack LIKE pattern operator

Checks whether a string matches a simple regular expression. The regular expression can contain the metasymbols % and _.

% indicates any quantity of any bytes (including zero characters).

_ indicates any one byte.

Use the backslash (\) for escaping metasymbols. See the note on escaping in the description of the 'match' function.

For regular expressions like %needle%, the code is more optimal and works as fast as the position function. For other regular expressions, the code is the same as for the 'match' function.

notLike(haystack, pattern), haystack NOT LIKE pattern operator

The same thing as 'like', but negative.

ngramDistance(haystack, needle)

Calculates the 4-gram distance between haystack and needle: counts the symmetric difference between two multisets of 4-grams and normalizes it by the sum of their cardinalities. Returns float number from 0 to 1 -- the closer to zero, the more strings are similar to each other. If the constant needle or haystack is more than 32Kb, throws an exception. If some of the non-constant haystack or needle strings are more than 32Kb, the distance is always one.

For case-insensitive search or/and in UTF-8 format use functions ngramDistanceCaseInsensitive, ngramDistanceUTF8, ngramDistanceCaseInsensitiveUTF8.

ngramSearch(haystack, needle)

Same as ngramDistance but calculates the non-symmetric difference between needle and haystack -- the number of n-grams from needle minus the common number of n-grams normalized by the number of needle n-grams. The closer to one, the more likely needle is in the haystack. Can be useful for fuzzy string search.

For case-insensitive search or/and in UTF-8 format use functions ngramSearchCaseInsensitive, ngramSearchUTF8, ngramSearchCaseInsensitiveUTF8.

!!! note "Note" For UTF-8 case we use 3-gram distance. All these are not perfectly fair n-gram distances. We use 2-byte hashes to hash n-grams and then calculate the (non-)symmetric difference between these hash tables -- collisions may occur. With UTF-8 case-insensitive format we do not use fair tolower function -- we zero the 5-th bit (starting from zero) of each codepoint byte and first bit of zeroth byte if bytes more than one -- this works for Latin and mostly for all Cyrillic letters.

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