sed: BRE syntax
5.3 Overview of basic regular expression syntax
===============================================
Here is a brief description of regular expression syntax as used in
'sed'.
'CHAR'
A single ordinary character matches itself.
'*'
Matches a sequence of zero or more instances of matches for the
preceding regular expression, which must be an ordinary character,
a special character preceded by '\', a '.', a grouped regexp (see
below), or a bracket expression. As a GNU extension, a postfixed
regular expression can also be followed by '*'; for example, 'a**'
is equivalent to 'a*'. POSIX 1003.1-2001 says that '*' stands for
itself when it appears at the start of a regular expression or
subexpression, but many nonGNU implementations do not support this
and portable scripts should instead use '\*' in these contexts.
'.'
Matches any character, including newline.
'^'
Matches the null string at beginning of the pattern space, i.e.
what appears after the circumflex must appear at the beginning of
the pattern space.
In most scripts, pattern space is initialized to the content of
each line (⇒How 'sed' works Execution Cycle.). So, it is a
useful simplification to think of '^#include' as matching only
lines where '#include' is the first thing on line--if there are
spaces before, for example, the match fails. This simplification
is valid as long as the original content of pattern space is not
modified, for example with an 's' command.
'^' acts as a special character only at the beginning of the
regular expression or subexpression (that is, after '\(' or '\|').
Portable scripts should avoid '^' at the beginning of a
subexpression, though, as POSIX allows implementations that treat
'^' as an ordinary character in that context.
'$'
It is the same as '^', but refers to end of pattern space. '$'
also acts as a special character only at the end of the regular
expression or subexpression (that is, before '\)' or '\|'), and its
use at the end of a subexpression is not portable.
'[LIST]'
'[^LIST]'
Matches any single character in LIST: for example, '[aeiou]'
matches all vowels. A list may include sequences like
'CHAR1-CHAR2', which matches any character between (inclusive)
CHAR1 and CHAR2. ⇒Character Classes and Bracket
Expressions.
'\+'
As '*', but matches one or more. It is a GNU extension.
'\?'
As '*', but only matches zero or one. It is a GNU extension.
'\{I\}'
As '*', but matches exactly I sequences (I is a decimal integer;
for portability, keep it between 0 and 255 inclusive).
'\{I,J\}'
Matches between I and J, inclusive, sequences.
'\{I,\}'
Matches more than or equal to I sequences.
'\(REGEXP\)'
Groups the inner REGEXP as a whole, this is used to:
* Apply postfix operators, like '\(abcd\)*': this will search
for zero or more whole sequences of 'abcd', while 'abcd*'
would search for 'abc' followed by zero or more occurrences of
'd'. Note that support for '\(abcd\)*' is required by POSIX
1003.1-2001, but many non-GNU implementations do not support
it and hence it is not universally portable.
* Use back references (see below).
'REGEXP1\|REGEXP2'
Matches either REGEXP1 or REGEXP2. Use parentheses to use complex
alternative regular expressions. The matching process tries each
alternative in turn, from left to right, and the first one that
succeeds is used. It is a GNU extension.
'REGEXP1REGEXP2'
Matches the concatenation of REGEXP1 and REGEXP2. Concatenation
binds more tightly than '\|', '^', and '$', but less tightly than
the other regular expression operators.
'\DIGIT'
Matches the DIGIT-th '\(...\)' parenthesized subexpression in the
regular expression. This is called a "back reference".
Subexpressions are implicitly numbered by counting occurrences of
'\(' left-to-right.
'\n'
Matches the newline character.
'\CHAR'
Matches CHAR, where CHAR is one of '$', '*', '.', '[', '\', or '^'.
Note that the only C-like backslash sequences that you can portably
assume to be interpreted are '\n' and '\\'; in particular '\t' is
not portable, and matches a 't' under most implementations of
'sed', rather than a tab character.
Note that the regular expression matcher is greedy, i.e., matches are
attempted from left to right and, if two or more matches are possible
starting at the same character, it selects the longest.
Examples:
'abcdef'
Matches 'abcdef'.
'a*b'
Matches zero or more 'a's followed by a single 'b'. For example,
'b' or 'aaaaab'.
'a\?b'
Matches 'b' or 'ab'.
'a\+b\+'
Matches one or more 'a's followed by one or more 'b's: 'ab' is the
shortest possible match, but other examples are 'aaaab' or 'abbbbb'
or 'aaaaaabbbbbbb'.
'.*'
'.\+'
These two both match all the characters in a string; however, the
first matches every string (including the empty string), while the
second matches only strings containing at least one character.
'^main.*(.*)'
This matches a string starting with 'main', followed by an opening
and closing parenthesis. The 'n', '(' and ')' need not be
adjacent.
'^#'
This matches a string beginning with '#'.
'\\$'
This matches a string ending with a single backslash. The regexp
contains two backslashes for escaping.
'\$'
Instead, this matches a string consisting of a single dollar sign,
because it is escaped.
'[a-zA-Z0-9]'
In the C locale, this matches any ASCII letters or digits.
'[^ '<TAB>']\+'
(Here '<TAB>' stands for a single tab character.) This matches a
string of one or more characters, none of which is a space or a
tab. Usually this means a word.
'^\(.*\)\n\1$'
This matches a string consisting of two equal substrings separated
by a newline.
'.\{9\}A$'
This matches nine characters followed by an 'A' at the end of a
line.
'^.\{15\}A'
This matches the start of a string that contains 16 characters, the
last of which is an 'A'.
Info Catalog
sed: BRE vs ERE
sed: sed regular expressions
sed: ERE syntax