The Backslash Character
=======================

  The `\' character has one of four different meanings, depending on
the context in which you use it and what syntax bits are set (
see Syntax Bits.).  It can: 1) stand for itself, 2) quote the next
character, 3) introduce an operator, or 4) do nothing.

  1. It stands for itself inside a list (see List Operators.) if
     the syntax bit `RE_BACKSLASH_ESCAPE_IN_LISTS' is not set.  For
     example, `[\]' would match `\'.

  2. It quotes (makes ordinary, if it's special) the next character
     when you use it either:

        * outside a list,(1) or

        * inside a list and the syntax bit
          `RE_BACKSLASH_ESCAPE_IN_LISTS' is set.

  3. It introduces an operator when followed by certain ordinary
     characters--sometimes only when certain syntax bits are set.  See
     the cases `RE_BK_PLUS_QM', `RE_NO_BK_BRACES', `RE_NO_BK_VAR',
     `RE_NO_BK_PARENS', `RE_NO_BK_REF' in See Syntax Bits.  Also:

        * `\b' represents the match-word-boundary operator (
see Match-word-boundary Operator.).

        * `\B' represents the match-within-word operator (
see Match-within-word Operator.).

        * `\<' represents the match-beginning-of-word operator
          (see Match-beginning-of-word Operator.).

        * `\>' represents the match-end-of-word operator (
see Match-end-of-word Operator.).

        * `\w' represents the match-word-constituent operator (
see Match-word-constituent Operator.).

        * `\W' represents the match-non-word-constituent operator
          (see Match-non-word-constituent Operator.).

        * `\`' represents the match-beginning-of-buffer operator and
          `\'' represents the match-end-of-buffer operator (
see Buffer Operators.).

        * If Regex was compiled with the C preprocessor symbol `emacs'
          defined, then `\sCLASS' represents the match-syntactic-class
          operator and `\SCLASS' represents the
          match-not-syntactic-class operator (*note Syntactic Class
          Operators::.).

  4. In all other cases, Regex ignores `\'.  For example, `\n' matches
     `n'.


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

  (1)  Sometimes you don't have to explicitly quote special characters
to make them ordinary.  For instance, most characters lose any special
meaning inside a list (see List Operators.).  In addition, if the
syntax bits `RE_CONTEXT_INVALID_OPS' and `RE_CONTEXT_INDEP_OPS' aren't
set, then (for historical reasons) the matcher considers special
characters ordinary if they are in contexts where the operations they
represent make no sense; for example, then the match-zero-or-more
operator (represented by `*') matches itself in the regular expression
`*foo' because there is no preceding expression on which it can
operate.  It is poor practice, however, to depend on this behavior; if
you want a special character to be ordinary outside a list, it's better
to always quote it, regardless.