Help Center/ GaussDB/ Developer Guide(Distributed_8.x)/ SQL Reference/ Functions and Operators/ Pattern Matching Operators
Updated on 2024-08-20 GMT+08:00
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Pattern Matching Operators

The database provides three independent methods for implementing pattern matching: SQL LIKE operator, SIMILAR TO operator, and POSIX-style regular expressions. Besides these basic operators, functions can be used to extract or replace matching substrings and to split a string at matching locations.

  • LIKE

    Description: Specifies whether the string matches the pattern string following 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.)

    Matching rules:
    1. This operator can succeed only when its pattern matches the entire string. If you want to match a sequence in any position within the string, the pattern must begin and end with a percent sign.
    2. An underscore (_) represents (matches) any single character. A percent sign (%) represents the wildcard character of any string.
    3. To match a literal underscore or percent sign, the respective character in pattern must be preceded by the escape character. The default escape character is one backslash but a different one can be selected by using the ESCAPE clause.
    4. To match with escape characters, enter two escape characters. For example, to write a pattern constant containing a backslash (\), you need to enter two backslashes in SQL statements.

      When standard_conforming_strings is set to off, any backslashes you write in literal string constants will need to be doubled. Therefore, writing a pattern matching a single backslash is actually going to write four backslashes in the statement. You can avoid this by selecting a different escape character by using ESCAPE, so that the backslash is no longer a special character of LIKE. But the backslash is still the special character of the character text analyzer. In this case, two backslashes are required. You can also select no escape character by writing "ESCAPE". This effectively disables the escape mechanism, but it does not eliminate the special meaning of underscore and percent signs in the pattern.

    5. The keyword ILIKE can be used instead of LIKE to make the match case-insensitive.
    6. Operator ~~ is equivalent to LIKE, and operator ~~* corresponds to ILIKE.

    Example:

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    gaussdb=# SELECT 'abc' LIKE 'abc' AS RESULT;
 result
-----------
 t
(1 row)

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    gaussdb=# SELECT 'abc' LIKE 'a%' AS RESULT;
 result
-----------
 t
(1 row)

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    gaussdb=# SELECT 'abc' LIKE '_b_' AS RESULT;
 result
-----------
 t
(1 row)

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    gaussdb=# SELECT 'abc' LIKE 'c' AS RESULT;
 result
-----------
 f
(1 row)
  • SIMILAR TO

    Description: Returns true or false depending on whether the pattern matches the given string. It is similar to LIKE, but differs in that SIMILAR TO uses the regular expression understanding pattern defined by the SQL standard.

    Matching rules:
    1. Similar to LIKE, this operator succeeds only when its pattern matches the entire string. If you want to match a sequence in any position within the string, the pattern must begin and end with a percent sign.
    2. An underscore (_) represents (matches) any single character. A percent sign (%) represents the wildcard character of any string.
    3. SIMILAR TO supports these pattern-matching metacharacters borrowed from POSIX-style regular expressions:

      Metacharacter

      Description

      |

      Specifies alternation (either of two alternatives).

      *

      Specifies repetition of the previous item zero or more times.

      +

      Specifies repetition of the previous item one or more times.

      ?

      Specifies repetition of the previous item zero or one time.

      {m}

      Specifies repetition of the previous item exactly m times.

      {m,}

      Specifies repetition of the previous item m or more times.

      {m,n}

      Specifies repetition of the previous item at least m times and does not exceed n times.

      ()

      Specifies that parentheses () can be used to group items into a single logical item.

      [...]

      Specifies a character class, just as in POSIX-style regular expressions.
    4. A preamble escape character disables the special meaning of any of these metacharacters. The rules for using escape characters are the same as those for LIKE.

    Regular expressions:

    The substring(string from pattern for escape) function extracts a substring that matches an SQL regular expression pattern.

    Example:

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    gaussdb=# SELECT 'abc' SIMILAR TO 'abc' AS RESULT;
 result
-----------
 t
(1 row)

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    gaussdb=# SELECT 'abc' SIMILAR TO 'a' AS RESULT;
 result
-----------
 f
(1 row)

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    gaussdb=# SELECT 'abc' SIMILAR TO '%(b|d)%' AS RESULT;
 result
-----------
 t
(1 row)

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    gaussdb=# SELECT 'abc' SIMILAR TO '(b|c)%'  AS RESULT;
 result
-----------
 f
(1 row)
  • POSIX-style regular expressions

    Description: A regular expression is a collation that is an abbreviated definition of a set of strings (a regular set). If a string is a member of a regular set described by a regular expression, the string matches the regular expression. POSIX-style regular expressions provide a more powerful means for pattern matching than the LIKE and SIMILAR TO operators. Table 1 Regular expression match operators lists all available operators for pattern matching using POSIX-style regular expressions.

    Table 1 Regular expression match operators

    Operator

    Description

    Example

    ~

    Matches a regular expression, which is case-sensitive.

    'thomas' ~ '.*thomas.*'

    ~*

    Matches a regular expression, which is case-insensitive.

    'thomas' ~* '.*Thomas.*'

    !~

    Does not match a regular expression, which is case-sensitive.

    'thomas' !~ '.*Thomas.*'

    !~*

    Does not match a regular expression, which is case-insensitive.

    'thomas' !~* '.*vadim.*'
    Matching rules:
    1. Unlike LIKE patterns, a regular expression is allowed to match anywhere within a string, unless the regular expression is explicitly anchored to the beginning or end of the string.
    2. Besides the metacharacters mentioned above, POSIX-style regular expressions also support the following pattern matching metacharacters:

      Metacharacter

      Description

      ^

      Specifies the match starting with a string.

      $

      Specifies the match at the end of a string.

      .

      Matches any single character.

    Regular expressions:

    POSIX-style regular expressions support the following functions:

    Example:

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    gaussdb=# SELECT 'abc' ~ 'Abc' AS RESULT;
result 
--------
 f
(1 row)

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    gaussdb=# SELECT 'abc' ~* 'Abc' AS RESULT;
 result 
--------
 t
(1 row)

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    gaussdb=# SELECT 'abc' !~ 'Abc' AS RESULT;
 result 
--------
 t
(1 row)

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    gaussdb=# SELECT 'abc'!~* 'Abc' AS RESULT;
 result 
--------
 f
(1 row)

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    gaussdb=# SELECT 'abc' ~ '^a' AS RESULT;
 result 
--------
 t
(1 row)

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    gaussdb=# SELECT 'abc' ~ '(b|d)'AS RESULT;
 result 
--------
 t
(1 row)

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    gaussdb=# SELECT 'abc' ~ '^(b|c)'AS RESULT;
 result 
--------
 f
(1 row)

    Although most regular expression searches can be executed quickly, they can still be artificially processed to require any length of time and any amount of memory. It is not recommended that you accept the regular expression search pattern from the non-security pattern source. If you must do this, you are advised to add the statement timeout limit. The search with the SIMILAR TO pattern has the same security risks, as SIMILAR TO provides many capabilities that are the same as those of the POSIX-style regular expression. The LIKE search is much simpler than the other two options. Therefore, it is more secure to accept the non-secure pattern source search.

  • [NOT] REGEXP/ [NOT] RLIKE

    Description: The REGEXP operator is used for regular expression matching and complies with POSIX-style regular expression matching rules. TRUE or FALSE is returned based on whether the pattern matches the given string.

    The following table describes the regular expression operators.

    Operator Name

    Description

    Syntax

    REGEXP

    Specifies whether a string matches the regular expression.

    expr REGEXP pat

    RLIKE

    Specifies whether a string matches the regular expression (same as REGEXP).

    expr RLIKE pat

    NOT REGEXP

    Specifies whether a string does not match the regular expression.

    expr NOT REGEXP pat

    NOT RLIKE

    Specifies whether the character string does not match the regular expression (same as NOT REGEXP).

    expr NOT RLIKE pat
    Matching rules:
    1. A regular expression allows you to match any position in a string unless the regular expression is explicitly concatenated at the beginning or end of the string.
    2. The pattern matching metacharacters supported by the REGEXP operator are the same as those supported by POSIX-style regular expressions.
    3. The operator supports the following escape character matching.

      Escape Character

      Description

      \b

      Backspace key.

      \f

      Form feed character, for example, C language.

      \n

      Newline character, for example, C language.

      \r

      Carriage return character, for example, C language.

      \t

      Horizontal tab character, for example, C language.

      \uwxyz

      Character whose hexadecimal value is 0xwxyz, where wxyz is four hexadecimal digits.

      \xhhh

      Character whose hexadecimal value is 0xhhh, where hhh is any sequence of hexadecimal digits.

      \xy

      Character whose octal value is 0xy, where xy is two octal digits.

      \xyz

      Character whose octal value is 0xyz, where xyz is three octal digits.
    4. Ranges for matching a pattern string: [a-dX] and [^a-dX].

      [a-dX] matches any characters of a, b, c, d, and X. [^a-dX] matches characters other than a, b, c, d, or X.

      The hyphen (-) between two characters forms a range, indicating that all characters in the range are matched. To include a right square bracket (]), it must follow the left square bracket ([). To include a hyphen (-), it must be after the left square bracket ([) or before the right square bracket (]). Any character that does not have any special meaning enclosed in square brackets ([]) matches itself.

      Example:

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      gaussdb=# SELECT 'abd' REGEXP 'a[bc]d' AS RESULT;
 result 
--------
 t
(1 row)

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      gaussdb=# SELECT 'aed' REGEXP 'a[^bc]d' AS RESULT;
 result 
--------
 t
(1 row)

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      gaussdb=# SELECT 'a-' REGEXP 'a[-b]' AS RESULT;
 result 
--------
 t
(1 row)

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      gaussdb=# SELECT 'aX]bc' REGEXP '^[]a-dXYZ]*$' AS RESULT;
 result 
--------
 t
(1 row)
    5. [.characters.] in the pattern string pat matches the collation of the element. In bracket expressions using quare brackets ([]), the collation used to proofread elements is matched. The character is a single character or a character name such as space. A complete list of character names can be found in the regex/regc_locale file.

      Example:

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      gaussdb=# SELECT ' ' REGEXP '[[.space.]]' AS RESULT;
 result 
--------
 t
(1 row)
    6. Character class matching the [=character_class=] character in the pattern string pat. It is written in the square bracket expression. [=character_class=] indicates the equivalence class. A character matches all characters with the same sort proofreading value, including itself. For example, if o and (+) are of the same class, [[=o=]], [[=(+)=]], and [o(+)] are synonyms. The same class cannot be used as an endpoint of a range.
    7. Character class matching the [:character_class:] character in the pattern string pat. It is written in square brackets ([]). [:character_class:] is used to match the characters that match the character class. Other class names may be provided for specific regions. The character class cannot be used as an endpoint of a range. The following table lists the standard class names. If the backslash (\) is involved, set the parameters according to the description.

      Character Class

      Description

      Character Range

      alnum

      Alphanumeric numeric character.

      [0-9a-zA-Z]

      alpha

      Alphanumeric character.

      [a-zA-Z]

      blank

      Blank character.

      [\t], indicating a blank character

      cntrl

      Control character.

      [\x01-\x1F]

      digit

      Digit character.

      [0-9]

      graph

      Graphic character.

      [^\x01-\x20]

      lower

      Lowercase character.

      [a-z]

      print

      Graphic character.

      [^\x01-\x20]

      punct

      Punctuation character.

      [-!"#$%&'( )*+,./:;<=>?@[\\]^_`{|}~]

      space

      Spaces, tabs, new lines, and carriage returns.

      [\n\r\t\x0B]

      upper

      Uppercase character.

      [A-Z]

      xdigit

      Hexadecimal numeric character.

      [0-9a-fA-F]

      Example:

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      gaussdb=# SELECT '\n' REGEXP '[[:space:]]' AS RESULT;
 result 
--------
 t
(1 row)
    8. Start and end matching of the [[:<:]], [[:>:]] matching string in the pattern string pat.

      Example:

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      gaussdb=# SELECT 'a word a' REGEXP '[[:<:]]word[[:>:]]' AS RESULT;
 result 
--------
 t
(1 row)
    9. For text instances that match special characters, add two backslashes (\) before the special characters (including (, ), ., ", ^, +, and ?). To match single quotation marks, you need to write \' in the source string to match \\\' in the pattern string. If the backslash (\) is involved, set the parameters according to the description.

      Example:

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      gaussdb=# SELECT 'a+b' REGEXP 'a\\+b' AS RESULT;
 result 
--------
 t
(1 row)
gaussdb=# SELECT '\'' REGEXP '\\\'' AS RESULT;
 result 
--------
 t
(1 row)
gaussdb=# SELECT '\\' REGEXP '\\\\' AS RESULT;
 result 
--------
 t
(1 row)
    10. When the case-sensitive character set and collation are set, regular expression matching is also case-sensitive.

      Example:

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      gaussdb=# SELECT 'abc' REGEXP 'ABC' COLLATE utf8mb4_bin AS RESULT;
 result 
--------
 f
(1 row)
    11. If the input parameter Expr or pat is NULL, an empty value is returned. 
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      gaussdb=# SELECT NULL REGEXP '*' AS RESULT;
 result 
--------

(1 row)
gaussdb=# SELECT '-' REGEXP NULL AS RESULT;
 result 
--------

(1 row)
      This operator is supported only in MySQL-compatible databases and is valid only when sql_compatibility is set to 'MYSQL'. b_format_version is set to '5.7'. In this case, the REGEXP operator is equivalent to the ~* operator, and the NOT REGEXP operator is equivalent to the !~* operator.
      • When b_format_dev_version is set to s2, standard_conforming_strings is set to off and escape_string_warning is set to off by default. In this case, any backslash written in the string constant must be written repeatedly. Therefore, writing a pattern matching a single backslash is actually going to write four backslashes in the statement.
      • This operator supports only the string type, text type, and other data types that can be implicitly converted to the string type and text type. The bytea type is not supported.
Parent topic: Functions and Operators