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.
Additionally, a pattern matching function,
substring
, is available, using either
SIMILAR TO
-style or POSIX-style regular
expressions.
If you have pattern matching needs that go beyond this, consider writing a user-defined function in Perl or Tcl.
string
LIKEpattern
[ESCAPEescape-character
]string
NOT LIKEpattern
[ESCAPEescape-character
]
Every pattern
defines a set of strings.
The LIKE
expression returns true if the
string
is contained in the set of
strings represented by 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 underscore, 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 string
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 matches always cover the entire
string. To match a sequence anywhere within a string, the
pattern must therefore 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 may be selected by
using the ESCAPE
clause. To match the escape
character itself, write two escape characters.
Note that the backslash already has a special meaning in string
literals, so to write a pattern constant that contains a backslash
you must write two backslashes in an SQL statement (assuming escape
string syntax is used). Thus, writing a pattern
that actually matches a literal backslash means writing four backslashes
in the statement. You can avoid this by selecting a different escape
character with ESCAPE
; then a backslash is not special
to LIKE
anymore. (But it is still special to the string
literal parser, so you still need two of them.)
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.
string
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 much like 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 practice, wherein the pattern
may 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.
Parentheses ()
may 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 bounded repetition (?
and {...}
)
are not provided, though they exist in POSIX. Also, the dot (.
)
is not a metacharacter.
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 to the entire data string, 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:
substring('foobar' from '%#"o_b#"%' for '#') oob substring('foobar' from '#"o_b#"%' for '#') NULL
Table 9.11, “Regular Expression Match Operators” lists the available operators for pattern matching using POSIX regular expressions.
Table 9.11. 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 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. (As always, remember to double backslashes written in literal
constant strings, assuming escape string syntax is used.)
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.
Some examples:
regexp_replace('foobarbaz', 'b..', 'X') fooXbaz regexp_replace('foobarbaz', 'b..', 'X', 'g') fooXX regexp_replace('foobarbaz', 'b(..)', E'X\\1Y', 'g') fooXarYXazY
PostgreSQL's regular expressions are implemented using a package written by Henry Spencer. Much of the description of regular expressions below is copied verbatim from his manual entry.
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 anyway
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.
The form of regular expressions accepted by
PostgreSQL can be chosen by setting the regex_flavor run-time parameter. The usual
setting is advanced
, but one might choose
extended
for maximum backwards compatibility with
pre-7.4 releases of PostgreSQL.
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 9.12, “Regular Expression Atoms”. The possible quantifiers and their meanings are shown in Table 9.13, “Regular Expression Quantifiers”.
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 may not be followed by a quantifier. The simple constraints are shown in Table 9.14, “Regular Expression Constraints”; some more constraints are described later.
Table 9.12. 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, “Bracket Expressions” 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, “Regular Expression Escapes”
(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 may not end with \
.
Remember that the backslash (\
) already has a special
meaning in PostgreSQL string literals.
To write a pattern constant that contains a backslash,
you must write two backslashes in the statement, assuming escape
string syntax is used.
Table 9.13. 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 may not 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, “Regular Expression Matching Rules” for more detail.
A quantifier cannot immediately follow another quantifier.
A quantifier cannot
begin an expression or subexpression or follow
^
or |
.
Table 9.14. 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) |
Lookahead constraints may not contain back references (see Section 9.7.3.3, “Regular Expression Escapes”), 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 (following a possible ^
). 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
a single element of the bracket expression's list. A bracket
expression containing a multiple-character collating element can thus
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 has no multicharacter 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
may not 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 may provide others. A character class may not 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 certainly 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 otherwise inconvenient characters in REs. They are shown in Table 9.15, “Regular Expression Character-Entry Escapes”.
Class-shorthand escapes provide shorthands for certain commonly-used character classes. They are shown in Table 9.16, “Regular Expression Class-Shorthand Escapes”.
A constraint escape is a constraint, matching the empty string if specific conditions are met, written as an escape. They are shown in Table 9.17, “Regular Expression Constraint Escapes”.
A back reference (\
n
) matches the
same string matched by the previous parenthesized subexpression specified
by the number n
(see Table 9.18, “Regular Expression Back References”). 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.
Keep in mind that an escape's leading \
will need to be
doubled when entering the pattern as an SQL string constant. For example:
'123' ~ E'^\\d{3}' true
Table 9.15. Regular Expression Character-Entry Escapes
Escape | Description |
---|---|
\a |
alert (bell) character, as in C |
\b |
backspace, as in C |
\B |
synonym for \ 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 UTF16 (Unicode, 16-bit) character U+ wxyz
in the local byte ordering |
\U stuvwxyz |
(where stuvwxyz is exactly eight hexadecimal
digits)
reserved for a somewhat-hypothetical Unicode extension to 32 bits
|
\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 |
\ 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
.
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 9.16. 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 9.17. Regular Expression Constraint Escapes
Escape | Description |
---|---|
\A |
matches only at the beginning of the string
(see Section 9.7.3.5, “Regular Expression Matching Rules” 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, “Regular Expression Matching Rules” for how this differs from
$ ) |
A word is defined as in the specification of
[[:<:]]
and [[:>:]]
above.
Constraint escapes are illegal within bracket expressions.
Table 9.18. 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 historical ambiguity between octal character-entry escapes and back references, which is resolved by 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 multidigit 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.
Normally the flavor of RE being used is determined by
regex_flavor
.
However, this can be overridden by a director prefix.
If an RE begins with ***:
,
the rest of the RE is taken as an ARE regardless of
regex_flavor
.
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 may 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 (including
both the RE flavor and case sensitivity).
The available option letters are
shown in Table 9.19, “ARE Embedded-Option Letters”.
Table 9.19. 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, “Regular Expression Matching Rules”) (overrides operator type) |
m |
historical synonym for n |
n |
newline-sensitive matching (see Section 9.7.3.5, “Regular Expression Matching Rules”) |
p |
partial newline-sensitive matching (see Section 9.7.3.5, “Regular Expression Matching Rules”) |
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, “Regular Expression Matching Rules”) |
x |
expanded syntax (see below) |
Embedded options take effect at the )
terminating the sequence.
They may 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 multicharacter 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
multicharacter 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.
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
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 \\
.
While these differences are unlikely to create a problem for most
applications, you can avoid them if necessary by
setting regex_flavor
to extended
.
BREs differ from EREs in several respects.
|
, +
, 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.