5.8 Escape Sequences - specifying special characters

Until this chapter, we have only encountered escapes of the form ‘\^’, which tell sed not to interpret the circumflex 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 escape⁶—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:

\a

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

\f

Produces or matches a form feed (ASCII 12).

\n

Produces or matches a newline (ASCII 10).

\r

Produces or matches a carriage return (ASCII 13).

\t

Produces or matches a horizontal tab (ASCII 9).

\v

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

\cx

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.

\dxxx

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

\oxxx

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

\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.