DuckDB offers pattern matching operators
(LIKE,
SIMILAR TO,
GLOB),
as well as support for regular expressions via functions.
Regular Expression Syntax
DuckDB uses the RE2 library as its regular expression engine. For the regular expression syntax, see the RE2 docs.
Functions
All functions accept an optional set of options.
| Name | Description |
|---|---|
regexp_extract(string, pattern[, group = 0][, options]) |
If string contains the regexp pattern, returns the capturing group specified by optional parameter group; otherwise, returns the empty string. The group must be a constant value. If no group is given, it defaults to 0. A set of optional options can be set. |
regexp_extract(string, pattern, name_list[, options]) |
If string contains the regexp pattern, returns the capturing groups as a struct with corresponding names from name_list; otherwise, returns a struct with the same keys and empty strings as values. |
regexp_extract_all(string, regex[, group = 0][, options]) |
Finds non-overlapping occurrences of regex in string and returns the corresponding values of group. |
regexp_full_match(string, regex[, options]) |
Returns true if the entire string matches the regex. |
regexp_matches(string, pattern[, options]) |
Returns true if string contains the regexp pattern, false otherwise. |
regexp_replace(string, pattern, replacement[, options]) |
If string contains the regexp pattern, replaces the matching part with replacement. |
regexp_split_to_array(string, regex[, options]) |
Alias of string_split_regex. Splits the string along the regex. |
regexp_split_to_table(string, regex[, options]) |
Splits the string along the regex and returns a row for each part. |
regexp_extract(string, pattern[, group = 0][, options])
| Description | If string contains the regexp pattern, returns the capturing group specified by optional parameter group; otherwise, returns the empty string. The group must be a constant value. If no group is given, it defaults to 0. A set of optional options can be set. |
| Example | regexp_extract('abc', '([a-z])(b)', 1) |
| Result | a |
regexp_extract(string, pattern, name_list[, options])
| Description | If string contains the regexp pattern, returns the capturing groups as a struct with corresponding names from name_list; otherwise, returns a struct with the same keys and empty strings as values. A set of optional options can be set. |
| Example | regexp_extract('2023-04-15', '(\d+)-(\d+)-(\d+)', ['y', 'm', 'd']) |
| Result | {'y':'2023', 'm':'04', 'd':'15'} |
regexp_extract_all(string, regex[, group = 0][, options])
| Description | Finds non-overlapping occurrences of regex in string and returns the corresponding values of group. A set of optional options can be set. |
| Example | regexp_extract_all('Peter: 33, Paul:14', '(\w+):\s*(\d+)', 2) |
| Result | [33, 14] |
regexp_full_match(string, regex[, options])
| Description | Returns true if the entire string matches the regex. A set of optional options can be set. |
| Example | regexp_full_match('anabanana', '(an)*') |
| Result | false |
regexp_matches(string, pattern[, options])
| Description | Returns true if string contains the regexp pattern, false otherwise. A set of optional options can be set. |
| Example | regexp_matches('anabanana', '(an)*') |
| Result | true |
regexp_replace(string, pattern, replacement[, options])
| Description | If string contains the regexp pattern, replaces the matching part with replacement. A set of optional options can be set. |
| Example | regexp_replace('hello', '[lo]', '-') |
| Result | he-lo |
regexp_split_to_array(string, regex[, options])
| Description | Alias of string_split_regex. Splits the string along the regex. A set of optional options can be set. |
| Example | regexp_split_to_array('hello world; 42', ';? ') |
| Result | ['hello', 'world', '42'] |
regexp_split_to_table(string, regex[, options])
| Description | Splits the string along the regex and returns a row for each part. A set of optional options can be set. |
| Example | regexp_split_to_table('hello world; 42', ';? ') |
| Result | Three rows: 'hello', 'world', '42' |
The regexp_matches function is similar to the SIMILAR TO operator, however, it does not require the entire string to match. Instead, regexp_matches returns true if the string merely contains the pattern (unless the special tokens ^ and $ are used to anchor the regular expression to the start and end of the string). Below are some examples:
SELECT regexp_matches('abc', 'abc'); -- true
SELECT regexp_matches('abc', '^abc$'); -- true
SELECT regexp_matches('abc', 'a'); -- true
SELECT regexp_matches('abc', '^a$'); -- false
SELECT regexp_matches('abc', '.*(b|d).*'); -- true
SELECT regexp_matches('abc', '(b|c).*'); -- true
SELECT regexp_matches('abc', '^(b|c).*'); -- false
SELECT regexp_matches('abc', '(?i)A'); -- true
SELECT regexp_matches('abc', 'A', 'i'); -- true
Options for Regular Expression Functions
The regex functions support the following options.
| Option | Description |
|---|---|
'c' |
case-sensitive matching |
'i' |
case-insensitive matching |
'l' |
match literals instead of regular expression tokens |
'm', 'n', 'p' |
newline sensitive matching |
'g' |
global replace, only available for regexp_replace |
's' |
non-newline sensitive matching |
For example:
SELECT regexp_matches('abcd', 'ABC', 'c'); -- false
SELECT regexp_matches('abcd', 'ABC', 'i'); -- true
SELECT regexp_matches('ab^/$cd', '^/$', 'l'); -- true
SELECT regexp_matches(E'hello\nworld', 'hello.world', 'p'); -- false
SELECT regexp_matches(E'hello\nworld', 'hello.world', 's'); -- true
Using regexp_matches
The regexp_matches operator will be optimized to the LIKE operator when possible. To achieve best performance, the 'c' option (case-sensitive matching) should be passed if applicable. Note that by default the RE2 library doesn't match the . character to newline.
| Original | Optimized equivalent |
|---|---|
regexp_matches('hello world', '^hello', 'c') |
prefix('hello world', 'hello') |
regexp_matches('hello world', 'world$', 'c') |
suffix('hello world', 'world') |
regexp_matches('hello world', 'hello.world', 'c') |
LIKE 'hello_world' |
regexp_matches('hello world', 'he.*rld', 'c') |
LIKE '%he%rld' |
Using regexp_replace
The regexp_replace function can be used to replace the part of a string that matches the regexp pattern with a replacement string. The notation \d (where d is a number indicating the group) can be used to refer to groups captured in the regular expression in the replacement string. Note that by default, regexp_replace only replaces the first occurrence of the regular expression. To replace all occurrences, use the global replace (g) flag.
Some examples for using regexp_replace:
SELECT regexp_replace('abc', '(b|c)', 'X'); -- aXc
SELECT regexp_replace('abc', '(b|c)', 'X', 'g'); -- aXX
SELECT regexp_replace('abc', '(b|c)', '\1\1\1\1'); -- abbbbc
SELECT regexp_replace('abc', '(.*)c', '\1e'); -- abe
SELECT regexp_replace('abc', '(a)(b)', '\2\1'); -- bac
Using regexp_extract
The regexp_extract function is used to extract a part of a string that matches the regexp pattern.
A specific capturing group within the pattern can be extracted using the group parameter. If group is not specified, it defaults to 0, extracting the first match with the whole pattern.
SELECT regexp_extract('abc', '.b.'); -- abc
SELECT regexp_extract('abc', '.b.', 0); -- abc
SELECT regexp_extract('abc', '.b.', 1); -- (empty)
SELECT regexp_extract('abc', '([a-z])(b)', 1); -- a
SELECT regexp_extract('abc', '([a-z])(b)', 2); -- b
The regexp_extract function also supports a name_list argument, which is a LIST of strings. Using name_list, the regexp_extract will return the corresponding capture groups as fields of a STRUCT:
SELECT regexp_extract('2023-04-15', '(\d+)-(\d+)-(\d+)', ['y', 'm', 'd']);
{'y': 2023, 'm': 04, 'd': 15}
SELECT regexp_extract('2023-04-15 07:59:56', '^(\d+)-(\d+)-(\d+) (\d+):(\d+):(\d+)', ['y', 'm', 'd']);
{'y': 2023, 'm': 04, 'd': 15}
SELECT regexp_extract('duckdb_0_7_1', '^(\w+)_(\d+)_(\d+)', ['tool', 'major', 'minor', 'fix']);
Binder Error: Not enough group names in regexp_extract
If the number of column names is less than the number of capture groups, then only the first groups are returned. If the number of column names is greater, then an error is generated.
Limitations
Regular expressions only support 9 capture groups: \1, \2, \3, …, \9.
Capture groups with two or more digits are not supported.