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.
LIKE
string
LIKEpattern
[ESCAPEescape-character
]string
NOT 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.
SIMILAR TO
Regular Expressionsstring
SIMILAR TOpattern
[ESCAPEescape-character
]string
NOT 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,
substring(
, provides
extraction of a substring that matches an SQL
regular expression pattern. As with string
from
pattern
for
escape-character
)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 that should be returned on success, the pattern must contain
two occurrences of the escape character followed by a double quote
("
).
The text matching the portion of the pattern
between these markers is returned.
Some examples, with #"
delimiting the return string:
substring('foobar' from '%#"o_b#"%' for '#') oob substring('foobar' from '#"o_b#"%' for '#') NULL
Table 9.14 lists the available operators for pattern matching using POSIX regular expressions.
Table 9.14. 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 9.22.
Some examples:
regexp_replace('foobarbaz', 'b..', 'X') fooXbaz regexp_replace('foobarbaz', 'b..', 'X', 'g') fooXX regexp_replace('foobarbaz', 'b(..)', E'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 9.22.
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 9.22, 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 9.22.
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', E'\\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', E'\\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', E'\\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 9.15. The possible quantifiers and their meanings are shown in Table 9.16.
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 9.17; some more constraints are described later.
Table 9.15. Regular Expression Atoms
Atom | Description |
---|---|
( re ) | (where re is any regular expression)
matches a match for
re , 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 k is a non-alphanumeric character)
matches that character taken as an ordinary character,
e.g., \\ matches a backslash character |
\ c | where c is alphanumeric
(possibly followed by other characters)
is an escape, see Section 9.7.3.3
(AREs only; in EREs and BREs, this matches c ) |
{ | when followed by a character other than a digit,
matches the left-brace character { ;
when followed by a digit, it is the beginning of a
bound (see below) |
x | where x is 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 9.16. 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 m matches of the atom |
{ m ,} | a sequence of m or more matches of the atom |
{ m , n } | a sequence of m through n
(inclusive) matches of the atom; m cannot exceed
n |
*? | 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 9.17. 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 re begins
(AREs only) |
(?! re ) | negative lookahead matches at any point
where no substring matching re begins
(AREs only) |
(?<= re ) | positive lookbehind matches at any point
where a substring matching re ends
(AREs only) |
(?<! re ) | negative lookbehind matches at any point
where no substring matching re ends
(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. Standard
character class names are: alnum
,
alpha
, blank
,
cntrl
, digit
,
graph
, lower
,
print
, punct
,
space
, upper
,
xdigit
. These stand for the character classes
defined in
ctype.
A locale can provide others. A character class cannot be used as
an endpoint of a range.
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
ctype)
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 9.18.
Class-shorthand escapes provide shorthands for certain commonly-used character classes. They are shown in Table 9.19.
A constraint escape is a constraint, matching the empty string if specific conditions are met, written as an escape. They are shown in Table 9.20.
A back reference (\
n
) matches the
same string matched by the previous parenthesized subexpression specified
by the number n
(see Table 9.21). 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 9.18. 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 |
\c X | (where X is any character) the character whose
low-order 5 bits are the same as those of
X , and whose other bits are all zero |
\e | the character whose collating-sequence name
is ESC ,
or failing that, the character with octal value 033 |
\f | form feed, as in C |
\n | newline, as in C |
\r | carriage return, as in C |
\t | horizontal tab, as in C |
\u wxyz | (where wxyz is exactly four hexadecimal digits)
the character whose hexadecimal value is
0x wxyz
|
\U stuvwxyz | (where stuvwxyz is exactly eight hexadecimal
digits)
the character whose hexadecimal value is
0x stuvwxyz
|
\v | vertical tab, as in C |
\x hhh | (where hhh is any sequence of hexadecimal
digits)
the character whose hexadecimal value is
0x hhh
(a single character no matter how many hexadecimal digits are used)
|
\0 | the character whose value is 0 (the null byte) |
\ xy | (where xy is exactly two octal digits,
and is not a back reference)
the character whose octal value is
0 xy |
\ xyz | (where xyz is exactly three octal digits,
and is not a back reference)
the character whose octal value is
0 xyz |
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 9.19. 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 9.20. 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 9.21. Regular Expression Back References
Escape | Description |
---|---|
\ m | (where m is a nonzero digit)
a back reference to the m 'th subexpression |
\ mnn | (where m is a nonzero digit, and
nn is some more digits, and the decimal value
mnn is not greater than the number of closing capturing
parentheses seen so far)
a back reference to the mnn '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 9.22.
Note that these same option letters are used in the flags
parameters of regex functions.
Table 9.22. 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.