Pattern Matching

Pattern Matching

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

LIKE

string LIKE pattern
string NOT LIKE pattern

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.

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

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.

SIMILAR TO

string SIMILAR TO pattern
string NOT SIMILAR TO pattern

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 β€” 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

Regular Expressions

| Function | Description | |:β€”|:β€”| | 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 |

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