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3.9 GNU Extensions for Escapes in Regular Expressions

Until this chapter, we have only encountered escapes of the form `\^', which tell sed not to interpret the caret as a special character, but rather to take it literally. For example, `\*' matches a single asterisk rather than zero or more backslashes.

This chapter introduces another kind of escape(5)---that is, escapes that are applied to a character or sequence of characters that ordinarily are taken literally, and that sed replaces with a special character. This provides a way of encoding non-printable characters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters in a sed script but when a script is being prepared in the shell or by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents:

The list of these escapes is:

Produces or matches a BEL character, that is an "alert" (ASCII 7).

Produces or matches a form feed (ASCII 12).

Produces or matches a new-line (ASCII 10).

Produces or matches a carriage return (ASCII 13).

Produces or matches a horizontal tab (ASCII 9).

Produces or matches a so called "vertical tab" (ASCII 11).

Produces or matches CONTROL-x, where x is any character. The precise effect of `\cx' is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus `\cz' becomes hex 1A, but `\c{' becomes hex 3B, while `\c;' becomes hex 7B.

Produces or matches a character whose decimal ASCII value is xxx.

Produces or matches a character whose octal ASCII value is xxx.

Produces or matches a character whose hexadecimal ASCII value is xx.

`\b' (backspace) was omitted because of the conflict with the existing "word boundary" meaning.

Other escapes match particular character class and are only valid in regular expressions:

Matches any whitespace character.

Matches any character that is not a whitespace character.

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

Matches any "non-word" character.

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