| PostgreSQL 8.4.22 Documentation | ||||
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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.
Tip: If you have pattern matching needs that go beyond this, consider writing a user-defined function in Perl or Tcl.
LIKEstring LIKE pattern [ESCAPE escape-character] string NOT LIKE pattern [ESCAPE escape-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, 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.
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, see Section
4.1.2.1). 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 backslash is still special to the string literal
parser, so you still need two of them to match a
backslash.)
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 TO pattern [ESCAPE escape-character] string NOT SIMILAR TO pattern [ESCAPE escape-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.
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 bounded repetition operators (? and {...}) are not provided, though they exist in POSIX. Also, the period (.) 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(string from pattern for escape-character), provides
extraction of a substring that matches an SQL regular
expression pattern. 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 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 9-11 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 POSIX pattern language is described in much greater detail below.
The substring function with
two parameters, substring(string from pattern), 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.
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. Other supported flags
are described in Table
9-19.
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_matches function
returns all of the captured substrings resulting from matching
a POSIX regular expression pattern. It has the syntax
regexp_matches(string, pattern [,
flags ]). If there is no
match to the pattern, the
function returns no rows. If there is a match, the function
returns 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). If the pattern does not contain any parenthesized
subexpressions, then the result is a single-element text array
containing the substring matching the whole pattern. The
flags parameter is an optional
text string containing zero or more single-letter flags that
change the function's behavior. Flag g
causes the function to find each match in the string, not only
the first one, and return a row for each such match. Other
supported flags are described in Table
9-19.
Some examples:
SELECT regexp_matches('foobarbequebaz', '(bar)(beque)');
regexp_matches
----------------
{bar,beque}
(1 row)
SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
regexp_matches
----------------
{bar,beque}
{bazil,barf}
(2 rows)
SELECT regexp_matches('foobarbequebaz', 'barbeque');
regexp_matches
----------------
{barbeque}
(1 row)
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
9-19.
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 jumped over the lazy dog', E'\\s+') AS foo;
foo
--------
the
quick
brown
fox
jumped
over
the
lazy
dog
(9 rows)
SELECT regexp_split_to_array('the quick brown fox jumped over the lazy dog', E'\\s+');
regexp_split_to_array
------------------------------------------------
{the,quick,brown,fox,jumped,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_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.
Note: 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 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. The possible quantifiers and their meanings are shown in Table 9-13.
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 9-14; 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 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 \.
Note: 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 (see Section 4.1.2.1).
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 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.
Note: A quantifier cannot immediately follow another quantifier, e.g., ** is invalid. 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 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.
Note: 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 9-15.
Class-shorthand escapes provide shorthands for certain commonly-used character classes. They are shown in Table 9-16.
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.
A back reference (\n) matches the same string matched by the previous parenthesized subexpression specified by the number n (see Table 9-18). 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.
Note: 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 backslash (\) to help reduce the need for backslash doubling |
| \cX | (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 |
| \uwxyz | (where wxyz is exactly four hexadecimal digits) the UTF16 (Unicode, 16-bit) character U+wxyz in the local byte ordering |
| \Ustuvwxyz | (where stuvwxyz is exactly eight hexadecimal digits) reserved for a hypothetical Unicode extension to 32 bits |
| \v | vertical tab, as in C |
| \xhhh | (where hhh is any sequence of hexadecimal digits) the character whose hexadecimal value is 0xhhh (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 0xy |
| \xyz | (where xyz is exactly three octal digits, and is not a back reference) the character whose octal value is 0xyz |
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 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 9-18. 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 |
Note: 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.
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 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 (including both the RE flavor and case sensitivity). The available option letters are shown in Table 9-19.
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) (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.
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. 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.