Pattern Matching
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Pattern Matching

There are four separate approaches to pattern matching provided by DuckDB: the traditional SQL LIKE operator, the more recent SIMILAR TO operator (added in SQL:1999), a GLOB operator, and POSIX-style regular expressions.

LIKE

The LIKE expression returns true if the string matches the supplied pattern. (As expected, the NOT LIKE expression returns false if LIKE returns true, and vice versa. An equivalent expression is NOT (string LIKE pattern).)

If pattern does not contain percent signs or underscores, then the pattern only represents the string itself; in that case LIKE acts like the equals operator. An underscore (_) in pattern stands for (matches) any single character; a percent sign (%) matches any sequence of zero or more characters.

LIKE pattern matching always covers the entire string. Therefore, if it’s desired to match a sequence anywhere within a string, the pattern must start and end with a percent sign.

Some examples:

'abc' LIKE 'abc' -- TRUE
'abc' LIKE 'a%'  -- TRUE
'abc' LIKE '_b_' -- TRUE
'abc' LIKE 'c'   -- FALSE
'abc' LIKE 'c%'  -- FALSE
'abc' LIKE '%c'  -- TRUE
'abc' NOT LIKE '%c'  -- FALSE

The keyword ILIKE can be used instead of LIKE to make the match case-insensitive according to the active locale.

'abc' ILIKE '%C' -- TRUE
'abc' NOT ILIKE '%C' -- FALSE

To search within a string for a character that is a wildcard (% or _), the pattern must use an ESCAPE clause and an escape character to indicate the wildcard should be treated as a literal character instead of a wildcard. See an example below.

Additionally, the function like_escape has the same functionality as a LIKE expression with an ESCAPE clause, but using function syntax. See the Text Functions Docs for details.

--Search for strings with 'a' then a literal percent sign then 'c'
'a%c' LIKE 'a$%c' ESCAPE '$'        -- TRUE
'azc' LIKE 'a$%c' ESCAPE '$'        -- FALSE

--Case insensitive ILIKE with ESCAPE
'A%c' ILIKE 'a$%c' ESCAPE '$';      --TRUE

There are also alternative characters that can be used as keywords in place of LIKE expressions. These enhance Postgres compatibility.

LIKE-style Postgres-style
LIKE ~~
NOT LIKE !~~
ILIKE ~~*
NOT ILIKE !~~*

SIMILAR TO

The SIMILAR TO operator returns true or false depending on whether its pattern matches the given string. It is similar to LIKE, except that it interprets the pattern using a regular expression. Like LIKE, the SIMILAR TO operator succeeds only if its pattern matches the entire string; this is unlike common regular expression behavior where the pattern can match any part of the string.

A regular expression is a character sequence that is an abbreviated definition of a set of strings (a regular set). A string is said to match a regular expression if it is a member of the regular set described by the regular expression. As with LIKE, pattern characters match string characters exactly unless they are special characters in the regular expression language — but regular expressions use different special characters than LIKE does.

Some examples:

'abc' SIMILAR TO 'abc'       -- TRUE
'abc' SIMILAR TO 'a'         -- FALSE
'abc' SIMILAR TO '.*(b|d).*' -- TRUE
'abc' SIMILAR TO '(b|c).*'   -- FALSE
'abc' NOT SIMILAR TO 'abc'   -- FALSE

There are also alternative characters that can be used as keywords in place of SIMILAR TO expressions. These follow POSIX syntax.

SIMILAR TO-style POSIX-style
SIMILAR TO ~
NOT SIMILAR TO !~

GLOB

The GLOB operator returns true or false if the string matches the GLOB pattern. The GLOB operator is most commonly used when searching for filenames that follow a specific pattern (for example a specific file extension). Use the question mark (?) wildcard to match any single character, and use the asterisk (*) to match zero or more characters. In addition, use bracket syntax ([ ]) to match any single character contained within the brackets, or within the character range specified by the brackets. An exclamation mark (!) may be used inside the first bracket to search for a character that is not contained within the brackets. To learn more, visit the Glob Programming Wikipedia page.

Some examples:

'best.txt' GLOB '*.txt'            -- TRUE
'best.txt' GLOB '????.txt'         -- TRUE
'best.txt' GLOB '?.txt'            -- FALSE
'best.txt' GLOB '[abc]est.txt'     -- TRUE
'best.txt' GLOB '[a-z]est.txt'     -- TRUE

-- The bracket syntax is case sensitive
'Best.txt' GLOB '[a-z]est.txt'     -- FALSE
'Best.txt' GLOB '[a-zA-Z]est.txt'  -- TRUE

-- The ! applies to all characters within the brackets
'Best.txt' GLOB '[!a-zA-Z]est.txt' -- FALSE

-- To negate a GLOB operator, negate the entire expression 
-- (NOT GLOB is not valid syntax)
NOT 'best.txt' GLOB '*.txt'        -- FALSE

Three tildes (~~~) may also be used in place of the GLOB keyword.

GLOB-style Symbolic-style
GLOB ~~~

Regular Expressions

Function Description
regexp_matches(string, pattern) returns TRUE if string contains the regexp pattern, FALSE otherwise
regexp_replace(string, pattern, replacement); if string contains the regexp pattern, replaces the matching part with replacement
regexp_extract(string, pattern [, idx]); if string contains the regexp pattern, returns the capturing group specified by optional parameter idx

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:

regexp_matches('abc', 'abc')       -- TRUE
regexp_matches('abc', '^abc$')     -- TRUE
regexp_matches('abc', 'a')         -- TRUE
regexp_matches('abc', '^a$')       -- FALSE
regexp_matches('abc', '.*(b|d).*') -- TRUE
regexp_matches('abc', '(b|c).*')   -- TRUE
regexp_matches('abc', '^(b|c).*')  -- FALSE

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. Below are some examples:

regexp_replace('abc', '(b|c)', 'X')        -- aXc
regexp_replace('abc', '(b|c)', '\1\1\1\1') -- abbbbc
regexp_replace('abc', '(.*)c', '\1e')      -- abe
regexp_replace('abc', '(a)(b)', '\2\1')    -- bac

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.

regexp_extract('abc', '.b.')     -- abc
regexp_extract('abc', '.b.', 0)  -- abc
regexp_extract('abc', '.b.', 1)  -- (empty)
regexp_extract('abc', '([a-z])(b)', 1) -- a
regexp_extract('abc', '([a-z])(b)', 2) -- b