regex: Regular Expressions as used in R
Description
This help page documents the regular expression patterns supported by
grep and related functions grepl, regexpr,
gregexpr, sub and gsub, as well as by
strsplit.Extended Regular Expressions
This section covers the regular expressions allowed in the default
mode of grep, regexpr, gregexpr, sub,
gsub and strsplit. They use an implementation of the
POSIX 1003.2 standard: that allows some scope for interpretation and
the interpretations here are those currently used by R. The
implementation supports some extensions to the standard. Regular expressions are constructed analogously to arithmetic
expressions, by using various operators to combine smaller
expressions. The whole expression matches zero or more characters
(read ‘character’ as ‘byte’ if useBytes = TRUE). The fundamental building blocks are the regular expressions that match
a single character. Most characters, including all letters and
digits, are regular expressions that match themselves. Any
metacharacter with special meaning may be quoted by preceding it with
a backslash. The metacharacters in extended regular expressions are
. \ | ( ) [ { ^ $ * + ?, but note that whether these have a
special meaning depends on the context. Escaping non-metacharacters with a backslash is
implementation-dependent. The current implementation interprets
\a as BEL, \e as ESC, \f as
FF, \n as LF, \r as CR and
\t as TAB. (Note that these will be interpreted by
R's parser in literal character strings.) A character class is a list of characters enclosed between
[ and ] which matches any single character in that list;
unless the first character of the list is the caret ^, when it
matches any character not in the list. For example, the
regular expression [0123456789] matches any single digit, and
[^abc] matches anything except the characters a,
b or c. A range of characters may be specified by
giving the first and last characters, separated by a hyphen. (Because
their interpretation is locale- and implementation-dependent,
character ranges are best avoided.) The only portable way to specify
all ASCII letters is to list them all as the character class
[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz]. (The
current implementation uses numerical order of the encoding.) Certain named classes of characters are predefined. Their
interpretation depends on the locale (see locales); the
interpretation below is that of the POSIX locale.
- [:alnum:]
- Alphanumeric characters: [:alpha:]
and [:digit:].
- [:alpha:]
- Alphabetic characters: [:lower:] and
[:upper:].
- [:blank:]
- Blank characters: space and tab, and
possibly other locale-dependent characters such as non-breaking
space.
- [:cntrl:]
-
Control characters. In ASCII, these characters have octal codes
000 through 037, and 177 (
DEL). In another character set,
these are the equivalent characters, if any. - [:digit:]
- Digits: 0 1 2 3 4 5 6 7 8 9.
- [:graph:]
- Graphical characters: [:alnum:] and
[:punct:].
- [:lower:]
- Lower-case letters in the current locale.
- [:print:]
-
Printable characters: [:alnum:], [:punct:] and space.
- [:punct:]
- Punctuation characters:
! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.
%'"` keep Emacs Rd mode happy - [:space:]
-
Space characters: tab, newline, vertical tab, form feed, carriage
return, space and possibly other locale-dependent characters.
- [:upper:]
- Upper-case letters in the current locale.
- [:xdigit:]
- Hexadecimal digits:
0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f.
For example, [[:alnum:]] means [0-9A-Za-z], except the
latter depends upon the locale and the character encoding, whereas the
former is independent of locale and character set. (Note that the
brackets in these class names are part of the symbolic names, and must
be included in addition to the brackets delimiting the bracket list.)
Most metacharacters lose their special meaning inside a character
class. To include a literal ], place it first in the list.
Similarly, to include a literal ^, place it anywhere but first.
Finally, to include a literal -, place it first or last (or,
for perl = TRUE only, precede it by a backslash). (Only
^ - \ ] are special inside character classes.) The period . matches any single character. The symbol
\w matches a ‘word’ character (a synonym for
[[:alnum:]_], an extension) and \W is its negation
([^[:alnum:]_]). Symbols \d, \s, \D
and \S denote the digit and space classes and their negations
(these are all extensions). The caret ^ and the dollar sign $ are metacharacters
that respectively match the empty string at the beginning and end of a
line. The symbols \< and \> match the empty string at
the beginning and end of a word. The symbol \b matches the
empty string at either edge of a word, and \B matches the
empty string provided it is not at an edge of a word. (The
interpretation of ‘word’ depends on the locale and
implementation: these are all extensions.) A regular expression may be followed by one of several repetition
quantifiers:
- ?
- The preceding item is optional and will be matched
at most once.
- *
- The preceding item will be matched zero or more
times.
- +
- The preceding item will be matched one or more
times.
- {n}
- The preceding item is matched exactly
n
times. - {n,}
- The preceding item is matched
n or more
times. - {n,m}
- The preceding item is matched at least
n
times, but not more than m times.
By default repetition is greedy, so the maximal possible number of
repeats is used. This can be changed to ‘minimal’ by appending
? to the quantifier. (There are further quantifiers that allow
approximate matching: see the TRE documentation.) Regular expressions may be concatenated; the resulting regular
expression matches any string formed by concatenating the substrings
that match the concatenated subexpressions. Two regular expressions may be joined by the infix operator |;
the resulting regular expression matches any string matching either
subexpression. For example, abba|cde matches either the
string abba or the string cde. Note that alternation
does not work inside character classes, where | has its literal
meaning. Repetition takes precedence over concatenation, which in turn takes
precedence over alternation. A whole subexpression may be enclosed in
parentheses to override these precedence rules. The backreference \N, where N = 1 ... 9, matches
the substring previously matched by the Nth parenthesized
subexpression of the regular expression. (This is an
extension for extended regular expressions: POSIX defines them only
for basic ones.)Perl-like Regular Expressions
The perl = TRUE argument to grep, regexpr,
gregexpr, sub, gsub and strsplit switches
to the PCRE library that implements regular expression pattern
matching using the same syntax and semantics as Perl 5.x,
with just a few differences. For complete details please consult the man pages for PCRE (and not
PCRE2), especially man pcrepattern and man
pcreapi), on your system or from the sources at
http://www.pcre.org. (The version in use can be found by
calling extSoftVersion. It need not be the version
described in the system's man page. As PCRE has been feature-frozen
for some time, the man pages at
http://www.pcre.org/original/doc/html/ should be a good match.) Perl regular expressions can be computed byte-by-byte or
(UTF-8) character-by-character: the latter is used in all multibyte
locales and if any of the inputs are marked as UTF-8 (see
Encoding). All the regular expressions described for extended regular expressions
are accepted except \< and \>: in Perl all backslashed
metacharacters are alphanumeric and backslashed symbols always are
interpreted as a literal character. { is not special if it
would be the start of an invalid interval specification. There can be
more than 9 backreferences (but the replacement in sub
can only refer to the first 9). Character ranges are interpreted in the numerical order of the
characters, either as bytes in a single-byte locale or as Unicode code
points in UTF-8 mode. So in either case [A-Za-z] specifies the
set of ASCII letters. In UTF-8 mode the named character classes only match ASCII characters:
see \p below for an alternative. The construct (?...) is used for Perl extensions in a variety
of ways depending on what immediately follows the ?. Perl-like matching can work in several modes, set by the options
(?i) (caseless, equivalent to Perl's /i), (?m)
(multiline, equivalent to Perl's /m), (?s) (single line,
so a dot matches all characters, even new lines: equivalent to Perl's
/s) and (?x) (extended, whitespace data characters are
ignored unless escaped and comments are allowed: equivalent to Perl's
/x). These can be concatenated, so for example, (?im)
sets caseless multiline matching. It is also possible to unset these
options by preceding the letter with a hyphen, and to combine setting
and unsetting such as (?im-sx). These settings can be applied
within patterns, and then apply to the remainder of the pattern.
Additional options not in Perl include (?U) to set
‘ungreedy’ mode (so matching is minimal unless ? is used
as part of the repetition quantifier, when it is greedy). Initially
none of these options are set. If you want to remove the special meaning from a sequence of
characters, you can do so by putting them between \Q and
\E. This is different from Perl in that $ and @ are
handled as literals in \Q...\E sequences in PCRE, whereas in
Perl, $ and @ cause variable interpolation. The escape sequences \d, \s and \w represent
any decimal digit, space character and ‘word’ character
(letter, digit or underscore in the current locale: in UTF-8 mode only
ASCII letters and digits are considered) respectively, and their
upper-case versions represent their negation. Vertical tab was not
regarded as a space character in a C locale before PCRE 8.34
(included in R 3.0.3). Sequences \h, \v, \H
and \V match horizontal and vertical space or the negation.
(In UTF-8 mode, these do match non-ASCII Unicode code points.) There are additional escape sequences: \cx is
cntrl-x for any x, \ddd is the
octal character (for up to three digits unless
interpretable as a backreference, as \1 to \7 always
are), and \xhh specifies a character by two hex digits.
In a UTF-8 locale, \x{h...} specifies a Unicode code point
by one or more hex digits. (Note that some of these will be
interpreted by R's parser in literal character strings.) Outside a character class, \A matches at the start of a
subject (even in multiline mode, unlike ^), \Z matches
at the end of a subject or before a newline at the end, \z
matches only at end of a subject. and \G matches at first
matching position in a subject (which is subtly different from Perl's
end of the previous match). \C matches a single
byte, including a newline, but its use is warned against. In UTF-8
mode, \R matches any Unicode newline character (not just CR),
and \X matches any number of Unicode characters that form an
extended Unicode sequence. In UTF-8 mode, some Unicode properties may be supported via
\p{xx} and \P{xx} which match characters with and
without property xx respectively. For a list of supported
properties see the PCRE documentation, but for example Lu is
‘upper case letter’ and Sc is ‘currency symbol’.
(This support depends on the PCRE library being compiled with
‘Unicode property support’ which can be checked via
pcre_config.) The sequence (?# marks the start of a comment which continues
up to the next closing parenthesis. Nested parentheses are not
permitted. The characters that make up a comment play no part at all in
the pattern matching. If the extended option is set, an unescaped # character outside
a character class introduces a comment that continues up to the next
newline character in the pattern. The pattern (?:...) groups characters just as parentheses do
but does not make a backreference. Patterns (?=...) and (?!...) are zero-width positive and
negative lookahead assertions: they match if an attempt to
match the … forward from the current position would succeed
(or not), but use up no characters in the string being processed.
Patterns (?<=...) and (?<!...) are the lookbehind
equivalents: they do not allow repetition quantifiers nor \C
in …. regexpr and gregexpr support ‘named capture’. If
groups are named, e.g., "(?<first>[A-Z][a-z]+)" then the
positions of the matches are also returned by name. (Named
backreferences are not supported by sub.) Atomic grouping, possessive qualifiers and conditional
and recursive patterns are not covered here.Details
A ‘regular expression’ is a pattern that describes a set of
strings. Two types of regular expressions are used in R,
extended regular expressions (the default) and
Perl-like regular expressions used by perl = TRUE.
There is a also fixed = TRUE which can be considered to use a
literal regular expression. Other functions which use regular expressions (often via the use of
grep) include apropos, browseEnv,
help.search, list.files and ls.
These will all use extended regular expressions. Patterns are described here as they would be printed by cat:
(do remember that backslashes need to be doubled when entering R
character strings, e.g. from the keyboard). Long regular expressions may or may not be accepted: the POSIX
standard only requires up to 256 bytes.