5.6 regular expression extensions

The following sequences have special meaning inside regular expressions (used in addresses and the s command).

These can be used in both basic and extended regular expressions (that is, with or without the -E/-r options).

\w

Matches any “word” character. A “word” character is any letter or digit or the underscore character.

$ echo "abc %-= def." | sed 's/\w/X/g'
XXX %-= XXX.

\W

Matches any “non-word” character.

$ echo "abc %-= def." | sed 's/\W/X/g'
abcXXXXXdefX

\b

Matches a word boundary; that is it matches if the character to the left is a “word” character and the character to the right is a “non-word” character, or vice-versa.

$ echo "abc %-= def." | sed 's/\b/X/g'
XabcX %-= XdefX.

\B

Matches everywhere but on a word boundary; that is it matches if the character to the left and the character to the right are either both “word” characters or both “non-word” characters.

$ echo "abc %-= def." | sed 's/\B/X/g'
aXbXc X%X-X=X dXeXf.X

\s

Matches whitespace characters (spaces and tabs). Newlines embedded in the pattern/hold spaces will also match:

$ echo "abc %-= def." | sed 's/\s/X/g'
abcX%-=Xdef.

\S

Matches non-whitespace characters.

$ echo "abc %-= def." | sed 's/\S/X/g'
XXX XXX XXXX

\<

Matches the beginning of a word.

$ echo "abc %-= def." | sed 's/\</X/g'
Xabc %-= Xdef.

\>

Matches the end of a word.

$ echo "abc %-= def." | sed 's/\>/X/g'
abcX %-= defX.

\`

Matches only at the start of pattern space. This is different from ^ in multi-line mode.

Compare the following two examples:

$ printf "a\nb\nc\n" | sed 'N;N;s/^/X/gm'
Xa
Xb
Xc

$ printf "a\nb\nc\n" | sed 'N;N;s/\`/X/gm'
Xa
b
c

\'

Matches only at the end of pattern space. This is different from $ in multi-line mode.