Two wildcard symbols are available for use in the search pattern:

• the percentage symbol (%) will match any sequence of zero or more characters in the tested value

• the underscore character (_) will match any single character in the tested value

If the tested value matches the pattern, taking into account wildcard symbols, the predicate is TRUE.

If the search string contains either of the wildcard symbols, the ESCAPE clause can be used to specify an escape character. The escape character must precede the % or _ symbol in the search string, to indicate that the symbol is to be interpreted as a literal character.

1. Find the numbers of departments whose names start with the word Software:

     |SELECT DEPT_NO
     |FROM DEPT
     |WHERE DEPT_NAME LIKE 'Software%';
   

   It is possible to use an index on the DEPT_NAME field if it exists.

   ⓘ

   About LIKE and the Optimizer

   Actually, the LIKE predicate does not use an index. However, if the predicate takes the form of LIKE 'string%', it will be converted to the STARTING WITH predicate, which will use an index. This optimization only works for literal patterns, not for parameters.

   So, if you need to search for the beginning of a string, it is recommended to use the Section 4.2.2.1.3, “STARTING WITH” predicate instead of the LIKE predicate.

2. Search for employees whose names consist of 5 letters, start with the letters Sm and end with th. The predicate will be true for such names as Smith and Smyth.

     |SELECT
     |  first_name
     |FROM
     |  employee
     |WHERE first_name LIKE 'Sm_th'
   

3. Search for all clients whose address contains the string Rostov:

     |SELECT *
     |FROM CUSTOMER
     |WHERE ADDRESS LIKE '%Rostov%'
   

   ⓘ

   Note

   If you need to do a case-insensitive search for something enclosed inside a string (LIKE '%Abc%'), use of the Section 4.2.2.1.4, “CONTAINING” predicate is recommended, in preference to the LIKE predicate.

4. Search for tables containing the underscore character in their names. The # character is used as the escape character:

     |SELECT
     |  RDB$RELATION_NAME
     |FROM RDB$RELATIONS
     |WHERE RDB$RELATION_NAME LIKE '%#_%' ESCAPE '#'
   

See alsoSTARTING WITH, CONTAINING, SIMILAR TO

The following syntax defines the SQL regular expression format. It is a complete and correct top-down definition. It is also highly formal, rather long and probably perfectly fit to discourage everybody who hasn’t already some experience with regular expressions (or with highly formal, rather long top-down definitions). Feel free to skip it and read the next section, Building Regular Expressions, which uses a bottom-up approach, aimed at the rest of us.

   |<regular expression> ::= <regular term> ['|' <regular term> ...]
   | 
   |<regular term> ::= <regular factor> ...
   | 
   |<regular factor> ::= <regular primary> [<quantifier>]
   | 
   |<quantifier> ::= ? | * | + | '{' <m> [,[<n>]] '}'
   | 
   |<m>, <n> ::= unsigned int, with <m> <= <n> if both present
   | 
   |<regular primary> ::=
   |    <character> | <character class> | %
   |  | (<regular expression>)
   | 
   |<character> ::= <escaped character> | <non-escaped character>
   | 
   |<escaped character> ::=
   |  <escape-char> <special character> | <escape-char> <escape-char>
   | 
   |<special character> ::= any of the characters []()|^-+*%_?{}
   | 
   |<non-escaped character> ::=
   |  any character that is not a <special character>
   |  and not equal to <escape-char> (if defined)
   | 
   |<character class> ::=
   |    '_' | '[' <member> ... ']' | '[^' <non-member> ... ']'
   |  | '[' <member> ... '^' <non-member> ... ']'
   | 
   |<member>, <non-member> ::= <character> | <range> | <predefined class>
   | 
   |<range> ::= <character>-<character>
   | 
   |<predefined class> ::= '[:' <predefined class name> ':]'
   | 
   |<predefined class name> ::=
   |  ALPHA | UPPER | LOWER | DIGIT | ALNUM | SPACE | WHITESPACE

In this section are the elements and rules for building SQL regular expressions.

Characters

Within regular expressions, most characters represent themselves. The only exceptions are the special characters below:

  |[ ] ( ) | ^ - + * % _ ? { }

... and the escape character, if it is defined.

A regular expression that contains no special character or escape characters matches only strings that are identical to itself (subject to the collation in use). That is, it functions just like the = operator:

  |'Apple' similar to 'Apple'  -- true
  |'Apples' similar to 'Apple' -- false
  |'Apple' similar to 'Apples' -- false
  |'APPLE' similar to 'Apple'  -- depends on collation

Wildcards

The known SQL wildcards _ and % match any single character and a string of any length, respectively:

  |'Birne' similar to 'B_rne'   -- true
  |'Birne' similar to 'B_ne'    -- false
  |'Birne' similar to 'B%ne'    -- true
  |'Birne' similar to 'Bir%ne%' -- true
  |'Birne' similar to 'Birr%ne' -- false

Notice how % also matches the empty string.

Character Classes

A bunch of characters enclosed in brackets define a character class. A character in the string matches a class in the pattern if the character is a member of the class:

  |'Citroen' similar to 'Cit[arju]oen'     -- true
  |'Citroen' similar to 'Ci[tr]oen'        -- false
  |'Citroen' similar to 'Ci[tr][tr]oen'    -- true

As can be seen from the second line, the class only matches a single character, not a sequence.

Within a class definition, two characters connected by a hyphen define a range. A range comprises the two endpoints and all the characters that lie between them in the active collation. Ranges can be placed anywhere in the class definition without special delimiters to keep them apart from the other elements.

  |'Datte' similar to 'Dat[q-u]e'          -- true
  |'Datte' similar to 'Dat[abq-uy]e'       -- true
  |'Datte' similar to 'Dat[bcg-km-pwz]e'   -- false

Predefined Character Classes

The following predefined character classes can also be used in a class definition:

[:ALPHA:]

Latin letters a..z and A..Z. With an accent-insensitive collation, this class also matches accented forms of these characters.

[:DIGIT:]

Decimal digits 0..9.

[:ALNUM:]

Union of [:ALPHA:] and [:DIGIT:].

[:UPPER:]

Uppercase Latin letters A..Z. Also matches lowercase with case-insensitive collation and accented forms with accent-insensitive collation.

[:LOWER:]

Lowercase Latin letters a..z. Also matches uppercase with case-insensitive collation and accented forms with accent-insensitive collation.

[:SPACE:]

Matches the space character (ASCII 32).

[:WHITESPACE:]

Matches horizontal tab (ASCII 9), linefeed (ASCII 10), vertical tab (ASCII 11), formfeed (ASCII 12), carriage return (ASCII 13) and space (ASCII 32).

Including a predefined class has the same effect as including all its members. Predefined classes are only allowed within class definitions. If you need to match against a predefined class and nothing more, place an extra pair of brackets around it.

  |'Erdbeere' similar to 'Erd[[:ALNUM:]]eere'     -- true
  |'Erdbeere' similar to 'Erd[[:DIGIT:]]eere'     -- false
  |'Erdbeere' similar to 'Erd[a[:SPACE:]b]eere'   -- true
  |'Erdbeere' similar to [[:ALPHA:]]              -- false
  |'E'        similar to [[:ALPHA:]]              -- true

If a class definition starts with a caret, everything that follows is excluded from the class. All other characters match:

  |'Framboise' similar to 'Fra[^ck-p]boise'       -- false
  |'Framboise' similar to 'Fr[^a][^a]boise'       -- false
  |'Framboise' similar to 'Fra[^[:DIGIT:]]boise'  -- true

If the caret is not placed at the start of the sequence, the class contains everything before the caret, except for the elements that also occur after the caret:

  |'Grapefruit' similar to 'Grap[a-m^f-i]fruit'   -- true
  |'Grapefruit' similar to 'Grap[abc^xyz]fruit'   -- false
  |'Grapefruit' similar to 'Grap[abc^de]fruit'    -- false
  |'Grapefruit' similar to 'Grap[abe^de]fruit'    -- false
  | 
  |'3' similar to '[[:DIGIT:]^4-8]'               -- true
  |'6' similar to '[[:DIGIT:]^4-8]'               -- false

Lastly, the already mentioned wildcard _ is a character class of its own, matching any single character.

Quantifiers

A question mark (?) immediately following a character or class indicates that the preceding item may occur 0 or 1 times in order to match:

  |'Hallon' similar to 'Hal?on'                   -- false
  |'Hallon' similar to 'Hal?lon'                  -- true
  |'Hallon' similar to 'Halll?on'                 -- true
  |'Hallon' similar to 'Hallll?on'                -- false
  |'Hallon' similar to 'Halx?lon'                 -- true
  |'Hallon' similar to 'H[a-c]?llon[x-z]?'        -- true

An asterisk (*) immediately following a character or class indicates that the preceding item may occur 0 or more times in order to match:

  |'Icaque' similar to 'Ica*que'                  -- true
  |'Icaque' similar to 'Icar*que'                 -- true
  |'Icaque' similar to 'I[a-c]*que'               -- true
  |'Icaque' similar to '_*'                       -- true
  |'Icaque' similar to '[[:ALPHA:]]*'             -- true
  |'Icaque' similar to 'Ica[xyz]*e'               -- false

A plus sign (+) immediately following a character or class indicates that the preceding item must occur 1 or more times in order to match:

  |'Jujube' similar to 'Ju_+'                     -- true
  |'Jujube' similar to 'Ju+jube'                  -- true
  |'Jujube' similar to 'Jujuber+'                 -- false
  |'Jujube' similar to 'J[jux]+be'                -- true
  |'Jujube' sililar to 'J[[:DIGIT:]]+ujube'       -- false

If a character or class is followed by a number enclosed in braces ({ and }), it must be repeated exactly that number of times in order to match:

  |'Kiwi' similar to 'Ki{2}wi'                    -- false
  |'Kiwi' similar to 'K[ipw]{2}i'                 -- true
  |'Kiwi' similar to 'K[ipw]{2}'                  -- false
  |'Kiwi' similar to 'K[ipw]{3}'                  -- true

If the number is followed by a comma (,), the item must be repeated at least that number of times in order to match:

  |'Limone' similar to 'Li{2,}mone'               -- false
  |'Limone' similar to 'Li{1,}mone'               -- true
  |'Limone' similar to 'Li[nezom]{2,}'            -- true

If the braces contain two numbers separated by a comma, the second number not smaller than the first, then the item must be repeated at least the first number and at most the second number of times in order to match:

  |'Mandarijn' similar to 'M[a-p]{2,5}rijn'       -- true
  |'Mandarijn' similar to 'M[a-p]{2,3}rijn'       -- false
  |'Mandarijn' similar to 'M[a-p]{2,3}arijn'      -- true

The quantifiers ?, * and + are shorthand for {0,1}, {0,} and {1,}, respectively.

OR-ing Terms

Regular expression terms can be OR’ed with the | operator. A match is made when the argument string matches at least one of the terms:

  |'Nektarin' similar to 'Nek|tarin'              -- false
  |'Nektarin' similar to 'Nektarin|Persika'       -- true
  |'Nektarin' similar to 'M_+|N_+|P_+'            -- true

Subexpressions

One or more parts of the regular expression can be grouped into subexpressions (also called subpatterns) by placing them between parentheses (( and )). A subexpression is a regular expression in its own right. It can contain all the elements allowed in a regular expression, and can also have quantifiers added to it.

  |'Orange' similar to 'O(ra|ri|ro)nge'           -- true
  |'Orange' similar to 'O(r[a-e])+nge'            -- true
  |'Orange' similar to 'O(ra){2,4}nge'            -- false
  |'Orange' similar to 'O(r(an|in)g|rong)?e'      -- true

Escaping Special Characters

In order to match against a character that is special in regular expressions, that character has to be escaped. There is no default escape character; rather, the user specifies one when needed:

  |'Peer (Poire)' similar to 'P[^ ]+ \(P[^ ]+\)' escape '\'    -- true
  |'Pera [Pear]'  similar to 'P[^ ]+ #[P[^ ]+#]' escape '#'    -- true
  |'Päron-äppledryck' similar to 'P%$-ä%' escape '$'           -- true
  |'Pärondryck' similar to 'P%--ä%' escape '-'                 -- false

The last line demonstrates that the escape character can also escape itself, if needed.