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

regexp_extract_all(string, regex[, group = 0])

regexp_extract(string, pattern, name_list)

regexp_extract(string, pattern[, idx])

regexp_full_match(string, regex)

regexp_matches(string, pattern)

regexp_replace(string, pattern, replacement)

regexp_split_to_array(string, regex)

regexp_split_to_table(string, regex)

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

Options for Regular Expression Functions

The regexp_matches and regexp_replace functions also support the following options.

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.

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 idx parameter. If idx 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

If ids is a LIST of strings, then 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']);
-- error

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.