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 that matches a single backslash actually involves writing four backslashes in the statement (you can avoid this by selecting a different escape character with 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, so you still need two backslashes.)

     In MySQL-compatible schema, it is also possible to select no escape character by writing ESCAPE ''. This effectively disables the escape mechanism, which makes it impossible to turn off the special meaning of underscore and percent signs in the schema.

  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 it 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.
     ()	Combines multiple items into a 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 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:

  • The substring(string from pattern) function provides a method for extracting a substring that matches the POSIX-style regular expression pattern.
  • The regexp_count(string text, pattern text [, position int [, flags text]]) function provides the function of obtaining the number of substrings that match the POSIX-style regular expression pattern.
  • The regexp_instr(string text, pattern text [, position int [, occurrence int [, return_opt int [, flags text]]]]) function is used to obtain the position of a substring that matches a POSIX-style regular expression pattern.
  • The regexp_substr(string text, pattern text [, position int [, occurrence int [, flags text]]]) function provides a method to extract a substring that matches a POSIX-style regular expression pattern.
  • The regexp_replace(string, pattern, replacement [,flags ]) function replaces the substring that matches the POSIX-style regular expression pattern with the new text.
  • The regexp_matches(string text, pattern text [, flags text]) function returns a text array consisting of all captured substrings that match a POSIX-style regular expression pattern.
  • The regexp_split_to_table(string text, pattern text [, flags text]) function splits a string using a POSIX-style regular expression pattern as a delimiter.
  • The regexp_split_to_array(string text, pattern text [, flags text ]) function behaves the same as regexp_split_to_table, except that regexp_split_to_array returns its result as an array of text.
    

    The regular expression split functions ignore zero-length matches, which occur at the beginning or end of a string or after the previous match. This is contrary to the strict definition of regular expression matching. The latter is implemented by regexp_matches, but the former is usually the most commonly used behavior in practice.

  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 the 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 is allowed to match anywhere within a string, unless the regular expression is explicitly anchored to 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, 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.
     \0	The GaussDB reports the error invalid byte sequence for encoding "UTF8": 0x00.
     \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. To match a text instance with 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, NULL 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 B-compatible databases and is valid only when sql_compatibility is set to 'B'. 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 double-written. 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.