There are three separate approaches to pattern matching provided
    by PostgreSQL: the traditional
    SQL LIKE operator, the
    more recent SIMILAR TO operator (added in
    SQL:1999), and POSIX-style regular
    expressions.  Aside from the basic “does this string match
    this pattern?” operators, functions are available to extract
    or replace matching substrings and to split a string at matching
    locations.
   
If you have pattern matching needs that go beyond this, consider writing a user-defined function in Perl or Tcl.
While most regular-expression searches can be executed very quickly, regular expressions can be contrived that take arbitrary amounts of time and memory to process. Be wary of accepting regular-expression search patterns from hostile sources. If you must do so, it is advisable to impose a statement timeout.
     Searches using SIMILAR TO patterns have the same
     security hazards, since SIMILAR TO provides many
     of the same capabilities as POSIX-style regular
     expressions.
    
     LIKE searches, being much simpler than the other
     two options, are safer to use with possibly-hostile pattern sources.
    
The pattern matching operators of all three kinds do not support nondeterministic collations. If required, apply a different collation to the expression to work around this limitation.
LIKEstringLIKEpattern[ESCAPEescape-character]stringNOT LIKEpattern[ESCAPEescape-character]
     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
    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.
   
    To match a literal underscore or percent sign without matching
    other characters, the respective character in
    pattern must be
    preceded by the escape character.  The default escape
    character is the backslash but a different one can be selected by
    using the ESCAPE clause.  To match the escape
    character itself, write two escape characters.
   
If you have standard_conforming_strings turned off, any backslashes you write in literal string constants will need to be doubled. See Section 4.1.2.1 for more information.
    It's 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 pattern.
   
    The key word ILIKE can be used instead of
    LIKE to make the match case-insensitive according
    to the active locale.  This is not in the SQL standard but is a
    PostgreSQL extension.
   
    The operator ~~ is equivalent to
    LIKE, and ~~* corresponds to
    ILIKE.  There are also
    !~~ and !~~* operators that
    represent NOT LIKE and NOT
    ILIKE, respectively.  All of these operators are
    PostgreSQL-specific.  You may see these
    operator names in EXPLAIN output and similar
    places, since the parser actually translates LIKE
    et al. to these operators.
   
    The phrases LIKE, ILIKE,
    NOT LIKE, and NOT ILIKE are
    generally treated as operators
    in PostgreSQL syntax; for example they can
    be used in expression
    operator ANY
    (subquery) constructs, although
    an ESCAPE clause cannot be included there.  In some
    obscure cases it may be necessary to use the underlying operator names
    instead.
   
    There is also the prefix operator ^@ and corresponding
    starts_with function which covers cases when only
    searching by beginning of the string is needed.
   
SIMILAR TO Regular ExpressionsstringSIMILAR TOpattern[ESCAPEescape-character]stringNOT SIMILAR TOpattern[ESCAPEescape-character]
    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 the SQL standard's definition of a
    regular expression.  SQL regular expressions are a curious cross
    between LIKE notation and common regular
    expression notation.
   
    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.
    Also like
    LIKE, SIMILAR TO uses
    _ and % as wildcard characters denoting
    any single character and any string, respectively (these are
    comparable to . and .* in POSIX regular
    expressions).
   
    In addition to these facilities borrowed from LIKE,
    SIMILAR TO supports these pattern-matching
    metacharacters borrowed from POSIX regular expressions:
   
      | denotes alternation (either of two alternatives).
     
      * denotes repetition of the previous item zero
      or more times.
     
      + denotes repetition of the previous item one
      or more times.
     
      ? denotes repetition of the previous item zero
      or one time.
     
      {m} denotes repetition
      of the previous item exactly m times.
     
      {m,} denotes repetition
      of the previous item m or more times.
     
      {m,n}
      denotes repetition of the previous item at least m and
      not more than n times.
     
      Parentheses () can be used to group items into
      a single logical item.
     
      A bracket expression [...] specifies a character
      class, just as in POSIX regular expressions.
     
    Notice that the period (.) is not a metacharacter
    for SIMILAR TO.
   
    As with LIKE, a backslash disables the special meaning
    of any of these metacharacters; or a different escape character can
    be specified with ESCAPE.
   
Some examples:
'abc' SIMILAR TO 'abc' true 'abc' SIMILAR TO 'a' false 'abc' SIMILAR TO '%(b|d)%' true 'abc' SIMILAR TO '(b|c)%' false
    The substring function with three parameters
    provides extraction of a substring that matches an SQL
    regular expression pattern.  The function can be written according
    to SQL99 syntax:
substring(stringfrompatternforescape-character)
or as a plain three-argument function:
substring(string,pattern,escape-character)
    As with SIMILAR TO, the
    specified pattern must match the entire data string, or else the
    function fails and returns null.  To indicate the part of the
    pattern for which the matching data sub-string is of interest,
    the pattern should contain
    two occurrences of the escape character followed by a double quote
    ("). 
    The text matching the portion of the pattern
    between these separators is returned when the match is successful.
   
    The escape-double-quote separators actually
    divide substring's pattern into three independent
    regular expressions; for example, a vertical bar (|)
    in any of the three sections affects only that section.  Also, the first
    and third of these regular expressions are defined to match the smallest
    possible amount of text, not the largest, when there is any ambiguity
    about how much of the data string matches which pattern.  (In POSIX
    parlance, the first and third regular expressions are forced to be
    non-greedy.)
   
As an extension to the SQL standard, PostgreSQL allows there to be just one escape-double-quote separator, in which case the third regular expression is taken as empty; or no separators, in which case the first and third regular expressions are taken as empty.
    Some examples, with #" delimiting the return string:
substring('foobar' from '%#"o_b#"%' for '#')   oob
substring('foobar' from '#"o_b#"%' for '#')    NULL
Table 9.15 lists the available operators for pattern matching using POSIX regular expressions.
Table 9.15. Regular Expression Match Operators
| Operator | Description | Example | 
|---|---|---|
| ~ | Matches regular expression, case sensitive | 'thomas' ~ '.*thomas.*' | 
| ~* | Matches regular expression, case insensitive | 'thomas' ~* '.*Thomas.*' | 
| !~ | Does not match regular expression, case sensitive | 'thomas' !~ '.*Thomas.*' | 
| !~* | Does not match regular expression, case insensitive | 'thomas' !~* '.*vadim.*' | 
     POSIX regular expressions provide a more
     powerful means for pattern matching than the LIKE and
     SIMILAR TO operators.
     Many Unix tools such as egrep,
     sed, or awk use a pattern
     matching language that is similar to the one described here.
    
     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 — but regular expressions use
     different special characters than LIKE does.
     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.
    
Some examples:
'abc' ~ 'abc' true 'abc' ~ '^a' true 'abc' ~ '(b|d)' true 'abc' ~ '^(b|c)' false
The POSIX pattern language is described in much greater detail below.
     The substring function with two parameters,
     substring(, provides extraction of a
     substring
     that matches a POSIX regular expression pattern.  It returns null if
     there is no match, otherwise the portion of the text that matched the
     pattern.  But if the pattern contains any parentheses, the portion
     of the text that matched the first parenthesized subexpression (the
     one whose left parenthesis comes first) is
     returned.  You can put parentheses around the whole expression
     if you want to use parentheses within it without triggering this
     exception.  If you need parentheses in the pattern before the
     subexpression you want to extract, see the non-capturing parentheses
     described below.
    string from
     pattern)
Some examples:
substring('foobar' from 'o.b')     oob
substring('foobar' from 'o(.)b')   o
     The regexp_replace function provides substitution of
     new text for substrings that match POSIX regular expression patterns.
     It has the syntax
     regexp_replace(source,
     pattern, replacement
     [, flags ]).
     The source string is returned unchanged if
     there is no match to the pattern.  If there is a
     match, the source string is returned with the
     replacement string substituted for the matching
     substring.  The replacement string can contain
     \n, where n is 1
     through 9, to indicate that the source substring matching the
     n'th parenthesized subexpression of the pattern should be
     inserted, and it can contain \& to indicate that the
     substring matching the entire pattern should be inserted.  Write
     \\ if you need to put a literal backslash in the replacement
     text.
     The flags parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Flag i specifies case-insensitive
     matching, while flag g specifies replacement of each matching
     substring rather than only the first one.  Supported flags (though
     not g) are
     described in Table 9.23.
    
Some examples:
regexp_replace('foobarbaz', 'b..', 'X')
                                   fooXbaz
regexp_replace('foobarbaz', 'b..', 'X', 'g')
                                   fooXX
regexp_replace('foobarbaz', 'b(..)', 'X\1Y', 'g')
                                   fooXarYXazY
     The regexp_match function returns a text array of
     captured substring(s) resulting from the first match of a POSIX
     regular expression pattern to a string.  It has the syntax
     regexp_match(string,
     pattern [, flags ]).
     If there is no match, the result is NULL.
     If a match is found, and the pattern contains no
     parenthesized subexpressions, then the result is a single-element text
     array containing the substring matching the whole pattern.
     If a match is found, and the pattern contains
     parenthesized subexpressions, then the result is a text array
     whose n'th element is the substring matching
     the n'th parenthesized subexpression of
     the pattern (not counting “non-capturing”
     parentheses; see below for details).
     The flags parameter is an optional text string
     containing zero or more single-letter flags that change the function's
     behavior.  Supported flags are described
     in Table 9.23.
    
Some examples:
SELECT regexp_match('foobarbequebaz', 'bar.*que');
 regexp_match
--------------
 {barbeque}
(1 row)
SELECT regexp_match('foobarbequebaz', '(bar)(beque)');
 regexp_match
--------------
 {bar,beque}
(1 row)
    In the common case where you just want the whole matching substring
    or NULL for no match, write something like
SELECT (regexp_match('foobarbequebaz', 'bar.*que'))[1];
 regexp_match
--------------
 barbeque
(1 row)
     The regexp_matches function returns a set of text arrays
     of captured substring(s) resulting from matching a POSIX regular
     expression pattern to a string.  It has the same syntax as
     regexp_match.
     This function returns no rows if there is no match, one row if there is
     a match and the g flag is not given, or N
     rows if there are N matches and the g flag
     is given.  Each returned row is a text array containing the whole
     matched substring or the substrings matching parenthesized
     subexpressions of the pattern, just as described above
     for regexp_match.
     regexp_matches accepts all the flags shown
     in Table 9.23, plus
     the g flag which commands it to return all matches, not
     just the first one.
    
Some examples:
SELECT regexp_matches('foo', 'not there');
 regexp_matches
----------------
(0 rows)
SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
 regexp_matches
----------------
 {bar,beque}
 {bazil,barf}
(2 rows)
     In most cases regexp_matches() should be used with
     the g flag, since if you only want the first match, it's
     easier and more efficient to use regexp_match().
     However, regexp_match() only exists
     in PostgreSQL version 10 and up.  When working in older
     versions, a common trick is to place a regexp_matches()
     call in a sub-select, for example:
SELECT col1, (SELECT regexp_matches(col2, '(bar)(beque)')) FROM tab;
     This produces a text array if there's a match, or NULL if
     not, the same as regexp_match() would do.  Without the
     sub-select, this query would produce no output at all for table rows
     without a match, which is typically not the desired behavior.
    
     The regexp_split_to_table function splits a string using a POSIX
     regular expression pattern as a delimiter.  It has the syntax
     regexp_split_to_table(string, pattern
     [, flags ]).
     If there is no match to the pattern, the function returns the
     string.  If there is at least one match, for each match it returns
     the text from the end of the last match (or the beginning of the string)
     to the beginning of the match.  When there are no more matches, it
     returns the text from the end of the last match to the end of the string.
     The flags parameter is an optional text string containing
     zero or more single-letter flags that change the function's behavior.
     regexp_split_to_table supports the flags described in
     Table 9.23.
    
     The regexp_split_to_array function behaves the same as
     regexp_split_to_table, except that regexp_split_to_array
     returns its result as an array of text.  It has the syntax
     regexp_split_to_array(string, pattern
     [, flags ]).
     The parameters are the same as for regexp_split_to_table.
    
Some examples:
SELECT foo FROM regexp_split_to_table('the quick brown fox jumps over the lazy dog', '\s+') AS foo;
  foo   
-------
 the    
 quick  
 brown  
 fox    
 jumps 
 over   
 the    
 lazy   
 dog    
(9 rows)
SELECT regexp_split_to_array('the quick brown fox jumps over the lazy dog', '\s+');
              regexp_split_to_array             
-----------------------------------------------
 {the,quick,brown,fox,jumps,over,the,lazy,dog}
(1 row)
SELECT foo FROM regexp_split_to_table('the quick brown fox', '\s*') AS foo;
 foo 
-----
 t         
 h         
 e         
 q         
 u         
 i         
 c         
 k         
 b         
 r         
 o         
 w         
 n         
 f         
 o         
 x         
(16 rows)
    As the last example demonstrates, the regexp split functions ignore
    zero-length matches that occur at the start or end of the string
    or immediately after a previous match.  This is contrary to the strict
    definition of regexp matching that is implemented by
    regexp_match and
    regexp_matches, but is usually the most convenient behavior
    in practice.  Other software systems such as Perl use similar definitions.
   
PostgreSQL's regular expressions are implemented using a software package written by Henry Spencer. Much of the description of regular expressions below is copied verbatim from his manual.
    Regular expressions (REs), as defined in
    POSIX 1003.2, come in two forms:
    extended REs or EREs
    (roughly those of egrep), and
    basic REs or BREs
    (roughly those of ed).
    PostgreSQL supports both forms, and
    also implements some extensions
    that are not in the POSIX standard, but have become widely used
    due to their availability in programming languages such as Perl and Tcl.
    REs using these non-POSIX extensions are called
    advanced REs or AREs
    in this documentation.  AREs are almost an exact superset of EREs,
    but BREs have several notational incompatibilities (as well as being
    much more limited).
    We first describe the ARE and ERE forms, noting features that apply
    only to AREs, and then describe how BREs differ.
   
PostgreSQL always initially presumes that a regular expression follows the ARE rules. However, the more limited ERE or BRE rules can be chosen by prepending an embedded option to the RE pattern, as described in Section 9.7.3.4. This can be useful for compatibility with applications that expect exactly the POSIX 1003.2 rules.
    A regular expression is defined as one or more
    branches, separated by
    |.  It matches anything that matches one of the
    branches.
   
A branch is zero or more quantified atoms or constraints, concatenated. It matches a match for the first, followed by a match for the second, etc; an empty branch matches the empty string.
A quantified atom is an atom possibly followed by a single quantifier. Without a quantifier, it matches a match for the atom. With a quantifier, it can match some number of matches of the atom. An atom can be any of the possibilities shown in Table 9.16. The possible quantifiers and their meanings are shown in Table 9.17.
A constraint matches an empty string, but matches only when specific conditions are met. A constraint can be used where an atom could be used, except it cannot be followed by a quantifier. The simple constraints are shown in Table 9.18; some more constraints are described later.
Table 9.16. Regular Expression Atoms
| Atom | Description | 
|---|---|
| (re) | (where reis any regular expression)
       matches a match forre, with the match noted for possible reporting | 
| (?:re) | as above, but the match is not noted for reporting (a “non-capturing” set of parentheses) (AREs only) | 
| . | matches any single character | 
| [chars] | a bracket expression,
       matching any one of the chars(see
       Section 9.7.3.2 for more detail) | 
| \k | (where kis a non-alphanumeric character)
       matches that character taken as an ordinary character,
       e.g.,\\matches a backslash character | 
| \c | where cis alphanumeric
       (possibly followed by other characters)
       is an escape, see Section 9.7.3.3
       (AREs only; in EREs and BREs, this matchesc) | 
| { | when followed by a character other than a digit,
       matches the left-brace character {;
       when followed by a digit, it is the beginning of abound(see below) | 
| x | where xis a single character with no other
       significance, matches that character | 
    An RE cannot end with a backslash (\).
   
If you have standard_conforming_strings turned off, any backslashes you write in literal string constants will need to be doubled. See Section 4.1.2.1 for more information.
Table 9.17. Regular Expression Quantifiers
| Quantifier | Matches | 
|---|---|
| * | a sequence of 0 or more matches of the atom | 
| + | a sequence of 1 or more matches of the atom | 
| ? | a sequence of 0 or 1 matches of the atom | 
| {m} | a sequence of exactly mmatches of the atom | 
| {m,} | a sequence of mor more matches of the atom | 
| {m,n} | a sequence of mthroughn(inclusive) matches of the atom;mcannot exceedn | 
| *? | non-greedy version of * | 
| +? | non-greedy version of + | 
| ?? | non-greedy version of ? | 
| {m}? | non-greedy version of {m} | 
| {m,}? | non-greedy version of {m,} | 
| {m,n}? | non-greedy version of {m,n} | 
    The forms using {...}
    are known as bounds.
    The numbers m and n within a bound are
    unsigned decimal integers with permissible values from 0 to 255 inclusive.
   
Non-greedy quantifiers (available in AREs only) match the same possibilities as their corresponding normal (greedy) counterparts, but prefer the smallest number rather than the largest number of matches. See Section 9.7.3.5 for more detail.
     A quantifier cannot immediately follow another quantifier, e.g.,
     ** is invalid.
     A quantifier cannot
     begin an expression or subexpression or follow
     ^ or |.
    
Table 9.18. Regular Expression Constraints
| Constraint | Description | 
|---|---|
| ^ | matches at the beginning of the string | 
| $ | matches at the end of the string | 
| (?=re) | positive lookahead matches at any point
       where a substring matching rebegins
       (AREs only) | 
| (?!re) | negative lookahead matches at any point
       where no substring matching rebegins
       (AREs only) | 
| (?<=re) | positive lookbehind matches at any point
       where a substring matching reends
       (AREs only) | 
| (?<!re) | negative lookbehind matches at any point
       where no substring matching reends
       (AREs only) | 
Lookahead and lookbehind constraints cannot contain back references (see Section 9.7.3.3), and all parentheses within them are considered non-capturing.
    A bracket expression is a list of
    characters enclosed in [].  It normally matches
    any single character from the list (but see below).  If the list
    begins with ^, it matches any single character
    not from the rest of the list.
    If two characters
    in the list are separated by -, this is
    shorthand for the full range of characters between those two
    (inclusive) in the collating sequence,
    e.g., [0-9] in ASCII matches
    any decimal digit.  It is illegal for two ranges to share an
    endpoint, e.g.,  a-c-e.  Ranges are very
    collating-sequence-dependent, so portable programs should avoid
    relying on them.
   
    To include a literal ] in the list, make it the
    first character (after ^, if that is used).  To
    include a literal -, make it the first or last
    character, or the second endpoint of a range.  To use a literal
    - as the first endpoint of a range, enclose it
    in [. and .] to make it a
    collating element (see below).  With the exception of these characters,
    some combinations using [
    (see next paragraphs), and escapes (AREs only), all other special
    characters lose their special significance within a bracket expression.
    In particular, \ is not special when following
    ERE or BRE rules, though it is special (as introducing an escape)
    in AREs.
   
    Within a bracket expression, a collating element (a character, a
    multiple-character sequence that collates as if it were a single
    character, or a collating-sequence name for either) enclosed in
    [. and .] stands for the
    sequence of characters of that collating element.  The sequence is
    treated as a single element of the bracket expression's list.  This
    allows a bracket
    expression containing a multiple-character collating element to
    match more than one character, e.g., if the collating sequence
    includes a ch collating element, then the RE
    [[.ch.]]*c matches the first five characters of
    chchcc.
   
PostgreSQL currently does not support multi-character collating elements. This information describes possible future behavior.
    Within a bracket expression, a collating element enclosed in
    [= and =] is an equivalence
    class, standing for the sequences of characters of all collating
    elements equivalent to that one, including itself.  (If there are
    no other equivalent collating elements, the treatment is as if the
    enclosing delimiters were [. and
    .].)  For example, if o and
    ^ are the members of an equivalence class, then
    [[=o=]], [[=^=]], and
    [o^] are all synonymous.  An equivalence class
    cannot be an endpoint of a range.
   
    Within a bracket expression, the name of a character class
    enclosed in [: and :] stands
    for the list of all characters belonging to that class.  A character
    class cannot be used as an endpoint of a range.
    The POSIX standard defines these character class
    names:
    alnum (letters and numeric digits),
    alpha (letters),
    blank (space and tab),
    cntrl (control characters),
    digit (numeric digits),
    graph (printable characters except space),
    lower (lower-case letters),
    print (printable characters including space),
    punct (punctuation),
    space (any white space),
    upper (upper-case letters),
    and xdigit (hexadecimal digits).
    The behavior of these standard character classes is generally
    consistent across platforms for characters in the 7-bit ASCII set.
    Whether a given non-ASCII character is considered to belong to one
    of these classes depends on the collation
    that is used for the regular-expression function or operator
    (see Section 23.2), or by default on the
    database's LC_CTYPE locale setting (see
    Section 23.1).  The classification of non-ASCII
    characters can vary across platforms even in similarly-named
    locales.  (But the C locale never considers any
    non-ASCII characters to belong to any of these classes.)
    In addition to these standard character
    classes, PostgreSQL defines
    the ascii character class, which contains exactly
    the 7-bit ASCII set.
   
    There are two special cases of bracket expressions:  the bracket
    expressions [[:<:]] and
    [[:>:]] are constraints,
    matching empty strings at the beginning
    and end of a word respectively.  A word is defined as a sequence
    of word characters that is neither preceded nor followed by word
    characters.  A word character is an alnum character (as
    defined by the POSIX character class described above)
    or an underscore.  This is an extension, compatible with but not
    specified by POSIX 1003.2, and should be used with
    caution in software intended to be portable to other systems.
    The constraint escapes described below are usually preferable; they
    are no more standard, but are easier to type.
   
    Escapes are special sequences beginning with \
    followed by an alphanumeric character. Escapes come in several varieties:
    character entry, class shorthands, constraint escapes, and back references.
    A \ followed by an alphanumeric character but not constituting
    a valid escape is illegal in AREs.
    In EREs, there are no escapes: outside a bracket expression,
    a \ followed by an alphanumeric character merely stands for
    that character as an ordinary character, and inside a bracket expression,
    \ is an ordinary character.
    (The latter is the one actual incompatibility between EREs and AREs.)
   
Character-entry escapes exist to make it easier to specify non-printing and other inconvenient characters in REs. They are shown in Table 9.19.
Class-shorthand escapes provide shorthands for certain commonly-used character classes. They are shown in Table 9.20.
A constraint escape is a constraint, matching the empty string if specific conditions are met, written as an escape. They are shown in Table 9.21.
    A back reference (\n) matches the
    same string matched by the previous parenthesized subexpression specified
    by the number n
    (see Table 9.22).  For example,
    ([bc])\1 matches bb or cc
    but not bc or cb.
    The subexpression must entirely precede the back reference in the RE.
    Subexpressions are numbered in the order of their leading parentheses.
    Non-capturing parentheses do not define subexpressions.
   
Table 9.19. Regular Expression Character-Entry Escapes
| Escape | Description | 
|---|---|
| \a | alert (bell) character, as in C | 
| \b | backspace, as in C | 
| \B | synonym for backslash ( \) to help reduce the need for backslash
       doubling | 
| \cX | (where Xis any character) the character whose
       low-order 5 bits are the same as those ofX, and whose other bits are all zero | 
| \e | the character whose collating-sequence name
       is ESC,
       or failing that, the character with octal value033 | 
| \f | form feed, as in C | 
| \n | newline, as in C | 
| \r | carriage return, as in C | 
| \t | horizontal tab, as in C | 
| \uwxyz | (where wxyzis exactly four hexadecimal digits)
       the character whose hexadecimal value is0xwxyz | 
| \Ustuvwxyz | (where stuvwxyzis exactly eight hexadecimal
       digits)
       the character whose hexadecimal value is0xstuvwxyz | 
| \v | vertical tab, as in C | 
| \xhhh | (where hhhis any sequence of hexadecimal
       digits)
       the character whose hexadecimal value is0xhhh(a single character no matter how many hexadecimal digits are used) | 
| \0 | the character whose value is 0(the null byte) | 
| \xy | (where xyis exactly two octal digits,
       and is not a back reference)
       the character whose octal value is0xy | 
| \xyz | (where xyzis exactly three octal digits,
       and is not a back reference)
       the character whose octal value is0xyz | 
    Hexadecimal digits are 0-9,
    a-f, and A-F.
    Octal digits are 0-7.
   
    Numeric character-entry escapes specifying values outside the ASCII range
    (0-127) have meanings dependent on the database encoding.  When the
    encoding is UTF-8, escape values are equivalent to Unicode code points,
    for example \u1234 means the character U+1234.
    For other multibyte encodings, character-entry escapes usually just
    specify the concatenation of the byte values for the character.  If the
    escape value does not correspond to any legal character in the database
    encoding, no error will be raised, but it will never match any data.
   
    The character-entry escapes are always taken as ordinary characters.
    For example, \135 is ] in ASCII, but
    \135 does not terminate a bracket expression.
   
Table 9.20. Regular Expression Class-Shorthand Escapes
| Escape | Description | 
|---|---|
| \d | [[:digit:]] | 
| \s | [[:space:]] | 
| \w | [[:alnum:]_](note underscore is included) | 
| \D | [^[:digit:]] | 
| \S | [^[:space:]] | 
| \W | [^[:alnum:]_](note underscore is included) | 
    Within bracket expressions, \d, \s,
    and \w lose their outer brackets,
    and \D, \S, and \W are illegal.
    (So, for example, [a-c\d] is equivalent to
    [a-c[:digit:]].
    Also, [a-c\D], which is equivalent to
    [a-c^[:digit:]], is illegal.)
   
Table 9.21. Regular Expression Constraint Escapes
| Escape | Description | 
|---|---|
| \A | matches only at the beginning of the string
       (see Section 9.7.3.5 for how this differs from ^) | 
| \m | matches only at the beginning of a word | 
| \M | matches only at the end of a word | 
| \y | matches only at the beginning or end of a word | 
| \Y | matches only at a point that is not the beginning or end of a word | 
| \Z | matches only at the end of the string
       (see Section 9.7.3.5 for how this differs from $) | 
    A word is defined as in the specification of
    [[:<:]] and [[:>:]] above.
    Constraint escapes are illegal within bracket expressions.
   
Table 9.22. Regular Expression Back References
| Escape | Description | 
|---|---|
| \m | (where mis a nonzero digit)
       a back reference to them'th subexpression | 
| \mnn | (where mis a nonzero digit, andnnis some more digits, and the decimal valuemnnis not greater than the number of closing capturing
       parentheses seen so far)
       a back reference to themnn'th subexpression | 
There is an inherent ambiguity between octal character-entry escapes and back references, which is resolved by the following heuristics, as hinted at above. A leading zero always indicates an octal escape. A single non-zero digit, not followed by another digit, is always taken as a back reference. A multi-digit sequence not starting with a zero is taken as a back reference if it comes after a suitable subexpression (i.e., the number is in the legal range for a back reference), and otherwise is taken as octal.
In addition to the main syntax described above, there are some special forms and miscellaneous syntactic facilities available.
    An RE can begin with one of two special director prefixes.
    If an RE begins with ***:,
    the rest of the RE is taken as an ARE.  (This normally has no effect in
    PostgreSQL, since REs are assumed to be AREs;
    but it does have an effect if ERE or BRE mode had been specified by
    the flags parameter to a regex function.)
    If an RE begins with ***=,
    the rest of the RE is taken to be a literal string,
    with all characters considered ordinary characters.
   
    An ARE can begin with embedded options:
    a sequence (?xyz)
    (where xyz is one or more alphabetic characters)
    specifies options affecting the rest of the RE.
    These options override any previously determined options —
    in particular, they can override the case-sensitivity behavior implied by
    a regex operator, or the flags parameter to a regex
    function.
    The available option letters are
    shown in Table 9.23.
    Note that these same option letters are used in the flags
    parameters of regex functions.
   
Table 9.23. ARE Embedded-Option Letters
| Option | Description | 
|---|---|
| b | rest of RE is a BRE | 
| c | case-sensitive matching (overrides operator type) | 
| e | rest of RE is an ERE | 
| i | case-insensitive matching (see Section 9.7.3.5) (overrides operator type) | 
| m | historical synonym for n | 
| n | newline-sensitive matching (see Section 9.7.3.5) | 
| p | partial newline-sensitive matching (see Section 9.7.3.5) | 
| q | rest of RE is a literal (“quoted”) string, all ordinary characters | 
| s | non-newline-sensitive matching (default) | 
| t | tight syntax (default; see below) | 
| w | inverse partial newline-sensitive (“weird”) matching (see Section 9.7.3.5) | 
| x | expanded syntax (see below) | 
    Embedded options take effect at the ) terminating the sequence.
    They can appear only at the start of an ARE (after the
    ***: director if any).
   
    In addition to the usual (tight) RE syntax, in which all
    characters are significant, there is an expanded syntax,
    available by specifying the embedded x option.
    In the expanded syntax,
    white-space characters in the RE are ignored, as are
    all characters between a #
    and the following newline (or the end of the RE).  This
    permits paragraphing and commenting a complex RE.
    There are three exceptions to that basic rule:
    
       a white-space character or # preceded by \ is
       retained
      
       white space or # within a bracket expression is retained
      
       white space and comments cannot appear within multi-character symbols,
       such as (?:
      
    For this purpose, white-space characters are blank, tab, newline, and
    any character that belongs to the space character class.
   
    Finally, in an ARE, outside bracket expressions, the sequence
    (?#ttt)
    (where ttt is any text not containing a ))
    is a comment, completely ignored.
    Again, this is not allowed between the characters of
    multi-character symbols, like (?:.
    Such comments are more a historical artifact than a useful facility,
    and their use is deprecated; use the expanded syntax instead.
   
    None of these metasyntax extensions is available if
    an initial ***= director
    has specified that the user's input be treated as a literal string
    rather than as an RE.
   
In the event that an RE could match more than one substring of a given string, the RE matches the one starting earliest in the string. If the RE could match more than one substring starting at that point, either the longest possible match or the shortest possible match will be taken, depending on whether the RE is greedy or non-greedy.
Whether an RE is greedy or not is determined by the following rules:
Most atoms, and all constraints, have no greediness attribute (because they cannot match variable amounts of text anyway).
Adding parentheses around an RE does not change its greediness.
       A quantified atom with a fixed-repetition quantifier
       ({m}
       or
       {m}?)
       has the same greediness (possibly none) as the atom itself.
      
       A quantified atom with other normal quantifiers (including
       {m,n}
       with m equal to n)
       is greedy (prefers longest match).
      
       A quantified atom with a non-greedy quantifier (including
       {m,n}?
       with m equal to n)
       is non-greedy (prefers shortest match).
      
       A branch — that is, an RE that has no top-level
       | operator — has the same greediness as the first
       quantified atom in it that has a greediness attribute.
      
       An RE consisting of two or more branches connected by the
       | operator is always greedy.
      
The above rules associate greediness attributes not only with individual quantified atoms, but with branches and entire REs that contain quantified atoms. What that means is that the matching is done in such a way that the branch, or whole RE, matches the longest or shortest possible substring as a whole. Once the length of the entire match is determined, the part of it that matches any particular subexpression is determined on the basis of the greediness attribute of that subexpression, with subexpressions starting earlier in the RE taking priority over ones starting later.
An example of what this means:
SELECT SUBSTRING('XY1234Z', 'Y*([0-9]{1,3})');
Result: 123
SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
Result: 1
    In the first case, the RE as a whole is greedy because Y*
    is greedy.  It can match beginning at the Y, and it matches
    the longest possible string starting there, i.e., Y123.
    The output is the parenthesized part of that, or 123.
    In the second case, the RE as a whole is non-greedy because Y*?
    is non-greedy.  It can match beginning at the Y, and it matches
    the shortest possible string starting there, i.e., Y1.
    The subexpression [0-9]{1,3} is greedy but it cannot change
    the decision as to the overall match length; so it is forced to match
    just 1.
   
In short, when an RE contains both greedy and non-greedy subexpressions, the total match length is either as long as possible or as short as possible, according to the attribute assigned to the whole RE. The attributes assigned to the subexpressions only affect how much of that match they are allowed to “eat” relative to each other.
    The quantifiers {1,1} and {1,1}?
    can be used to force greediness or non-greediness, respectively,
    on a subexpression or a whole RE.
    This is useful when you need the whole RE to have a greediness attribute
    different from what's deduced from its elements.  As an example,
    suppose that we are trying to separate a string containing some digits
    into the digits and the parts before and after them.  We might try to
    do that like this:
SELECT regexp_match('abc01234xyz', '(.*)(\d+)(.*)');
Result: {abc0123,4,xyz}
    That didn't work: the first .* is greedy so
    it “eats” as much as it can, leaving the \d+ to
    match at the last possible place, the last digit.  We might try to fix
    that by making it non-greedy:
SELECT regexp_match('abc01234xyz', '(.*?)(\d+)(.*)');
Result: {abc,0,""}
That didn't work either, because now the RE as a whole is non-greedy and so it ends the overall match as soon as possible. We can get what we want by forcing the RE as a whole to be greedy:
SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
Result: {abc,01234,xyz}
Controlling the RE's overall greediness separately from its components' greediness allows great flexibility in handling variable-length patterns.
    When deciding what is a longer or shorter match,
    match lengths are measured in characters, not collating elements.
    An empty string is considered longer than no match at all.
    For example:
    bb*
    matches the three middle characters of abbbc;
    (week|wee)(night|knights)
    matches all ten characters of weeknights;
    when (.*).*
    is matched against abc the parenthesized subexpression
    matches all three characters; and when
    (a*)* is matched against bc
    both the whole RE and the parenthesized
    subexpression match an empty string.
   
    If case-independent matching is specified,
    the effect is much as if all case distinctions had vanished from the
    alphabet.
    When an alphabetic that exists in multiple cases appears as an
    ordinary character outside a bracket expression, it is effectively
    transformed into a bracket expression containing both cases,
    e.g., x becomes [xX].
    When it appears inside a bracket expression, all case counterparts
    of it are added to the bracket expression, e.g.,
    [x] becomes [xX]
    and [^x] becomes [^xX].
   
    If newline-sensitive matching is specified, .
    and bracket expressions using ^
    will never match the newline character
    (so that matches will never cross newlines unless the RE
    explicitly arranges it)
    and ^ and $
    will match the empty string after and before a newline
    respectively, in addition to matching at beginning and end of string
    respectively.
    But the ARE escapes \A and \Z
    continue to match beginning or end of string only.
   
    If partial newline-sensitive matching is specified,
    this affects . and bracket expressions
    as with newline-sensitive matching, but not ^
    and $.
   
    If inverse partial newline-sensitive matching is specified,
    this affects ^ and $
    as with newline-sensitive matching, but not .
    and bracket expressions.
    This isn't very useful but is provided for symmetry.
   
No particular limit is imposed on the length of REs in this implementation. However, programs intended to be highly portable should not employ REs longer than 256 bytes, as a POSIX-compliant implementation can refuse to accept such REs.
    The only feature of AREs that is actually incompatible with
    POSIX EREs is that \ does not lose its special
    significance inside bracket expressions.
    All other ARE features use syntax which is illegal or has
    undefined or unspecified effects in POSIX EREs;
    the *** syntax of directors likewise is outside the POSIX
    syntax for both BREs and EREs.
   
    Many of the ARE extensions are borrowed from Perl, but some have
    been changed to clean them up, and a few Perl extensions are not present.
    Incompatibilities of note include \b, \B,
    the lack of special treatment for a trailing newline,
    the addition of complemented bracket expressions to the things
    affected by newline-sensitive matching,
    the restrictions on parentheses and back references in lookahead/lookbehind
    constraints, and the longest/shortest-match (rather than first-match)
    matching semantics.
   
Two significant incompatibilities exist between AREs and the ERE syntax recognized by pre-7.4 releases of PostgreSQL:
       In AREs, \ followed by an alphanumeric character is either
       an escape or an error, while in previous releases, it was just another
       way of writing the alphanumeric.
       This should not be much of a problem because there was no reason to
       write such a sequence in earlier releases.
      
       In AREs, \ remains a special character within
       [], so a literal \ within a bracket
       expression must be written \\.
      
    BREs differ from EREs in several respects.
    In BREs, |, +, and ?
    are ordinary characters and there is no equivalent
    for their functionality.
    The delimiters for bounds are
    \{ and \},
    with { and }
    by themselves ordinary characters.
    The parentheses for nested subexpressions are
    \( and \),
    with ( and ) by themselves ordinary characters.
    ^ is an ordinary character except at the beginning of the
    RE or the beginning of a parenthesized subexpression,
    $ is an ordinary character except at the end of the
    RE or the end of a parenthesized subexpression,
    and * is an ordinary character if it appears at the beginning
    of the RE or the beginning of a parenthesized subexpression
    (after a possible leading ^).
    Finally, single-digit back references are available, and
    \< and \>
    are synonyms for
    [[:<:]] and [[:>:]]
    respectively; no other escapes are available in BREs.
   
LIKE_REGEX)
     Since SQL:2008, the SQL standard includes
     a LIKE_REGEX operator that performs pattern
     matching according to the XQuery regular expression
     standard.  PostgreSQL does not yet
     implement this operator, but you can get very similar behavior using
     the regexp_match() function, since XQuery
     regular expressions are quite close to the ARE syntax described above.
    
Notable differences between the existing POSIX-based regular-expression feature and XQuery regular expressions include:
        XQuery character class subtraction is not supported.  An example of
        this feature is using the following to match only English
        consonants: [a-z-[aeiou]].
       
        XQuery character class shorthands \c,
        \C, \i,
        and \I are not supported.
       
        XQuery character class elements
        using \p{UnicodeProperty} or the
        inverse \P{UnicodeProperty} are not supported.
       
        POSIX interprets character classes such as \w
        (see Table 9.20)
        according to the prevailing locale (which you can control by
        attaching a COLLATE clause to the operator or
        function).  XQuery specifies these classes by reference to Unicode
        character properties, so equivalent behavior is obtained only with
        a locale that follows the Unicode rules.
       
        The SQL standard (not XQuery itself) attempts to cater for more
        variants of “newline” than POSIX does.  The
        newline-sensitive matching options described above consider only
        ASCII NL (\n) to be a newline, but SQL would have
        us treat CR (\r), CRLF (\r\n)
        (a Windows-style newline), and some Unicode-only characters like
        LINE SEPARATOR (U+2028) as newlines as well.
        Notably, . and \s should
        count \r\n as one character not two according to
        SQL.
       
        Of the character-entry escapes described in
        Table 9.19,
        XQuery supports only \n, \r,
        and \t.
       
        XQuery does not support
        the [: syntax
        for character classes within bracket expressions.
       name:]
XQuery does not have lookahead or lookbehind constraints, nor any of the constraint escapes described in Table 9.21.
The metasyntax forms described in Section 9.7.3.4 do not exist in XQuery.
        The regular expression flag letters defined by XQuery are
        related to but not the same as the option letters for POSIX
        (Table 9.23).  While the
        i and q options behave the
        same, others do not:
        
           XQuery's s (allow dot to match newline)
           and m (allow ^
           and $ to match at newlines) flags provide
           access to the same behaviors as
           POSIX's n, p
           and w flags, but they
           do not match the behavior of
           POSIX's s and m flags.
           Note in particular that dot-matches-newline is the default
           behavior in POSIX but not XQuery.
          
           XQuery's x (ignore whitespace in pattern) flag
           is noticeably different from POSIX's expanded-mode flag.
           POSIX's x flag also
           allows # to begin a comment in the pattern,
           and POSIX will not ignore a whitespace character after a
           backslash.