File: sed.info,  Node: Escapes,  Next: Locale Considerations,  Prev: Back-references and Subexpressions,  Up: sed regular expressions

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 escape(1)--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.

5.8.1 Escaping Precedence
-------------------------

GNU ‘sed’ processes escape sequences _before_ passing the text onto the
regular-expression matching of the ‘s///’ command and address matching.
Thus the following two commands are equivalent (‘0x5e’ is the
hexadecimal ASCII value of the character ‘^’):

     $ echo 'a^c' | sed 's/^/b/'
     ba^c

     $ echo 'a^c' | sed 's/\x5e/b/'
     ba^c

   As are the following (‘0x5b’,‘0x5d’ are the hexadecimal ASCII values
of ‘[’,‘]’, respectively):

     $ echo abc | sed 's/[a]/x/'
     xbc
     $ echo abc | sed 's/\x5ba\x5d/x/'
     xbc

   However it is recommended to avoid such special characters due to
unexpected edge-cases.  For example, the following are not equivalent:

     $ echo 'a^c' | sed 's/\^/b/'
     abc

     $ echo 'a^c' | sed 's/\\\x5e/b/'
     a^c

   ---------- Footnotes ----------

   (1) All the escapes introduced here are GNU extensions, with the
exception of ‘\n’.  In basic regular expression mode, setting
‘POSIXLY_CORRECT’ disables them inside bracket expressions.