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SIMILAR TO
Syntax
string-expression [NOT] SIMILAR TO <pattern> [ESCAPE <escape-char>]

<pattern>     ::= an SQL regular expression
<escape-char> ::= a single character

SIMILAR TO matches a string against an SQL regular expression pattern.Unlike in some other languages, the pattern must match the entire string to succeed — matching a substring is not enough.If any operand is NULL, the result is NULL.Otherwise, the result is TRUE or FALSE.

If a literal pattern is used, and it doesn’t start with a wildcard or other special regex character, SIMILAR TO can use an index.

Escaping Special Characters

To match against a character that is special in regular expressions, that character has to be escaped.There is no default escape character;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.

Syntax: SQL Regular Expressions

The following syntax defines the SQL regular expression format.It is a complete and correct top-down definition.It is also highly formal and long, and may be daunting to anyone 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
Building Regular Expressions

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