Syntax Bits
===========

  In any particular syntax for regular expressions, some characters are
always special, others are sometimes special, and others are never
special.  The particular syntax that Regex recognizes for a given
regular expression depends on the value in the `syntax' field of the
pattern buffer of that regular expression.

  You get a pattern buffer by compiling a regular expression.  
See GNU Pattern Buffers, and See POSIX Pattern Buffers, for more
information on pattern buffers.  *Note GNU Regular Expression
Compiling::, See POSIX Regular Expression Compiling, and *Note BSD
Regular Expression Compiling::, for more information on compiling.

  Regex considers the value of the `syntax' field to be a collection of
bits; we refer to these bits as "syntax bits".  In most cases, they
affect what characters represent what operators.  We describe the
meanings of the operators to which we refer in See Common Operators,
See GNU Operators, and See GNU Emacs Operators.

  For reference, here is the complete list of syntax bits, in
alphabetical order:

`RE_BACKSLASH_ESCAPE_IN_LISTS'
     If this bit is set, then `\' inside a list (see List Operators.
     quotes (makes ordinary, if it's special) the following character;
     if this bit isn't set, then `\' is an ordinary character inside
     lists.  (See The Backslash Character, for what `\' does
     outside of lists.)

`RE_BK_PLUS_QM'
     If this bit is set, then `\+' represents the match-one-or-more
     operator and `\?' represents the match-zero-or-more operator; if
     this bit isn't set, then `+' represents the match-one-or-more
     operator and `?' represents the match-zero-or-one operator.  This
     bit is irrelevant if `RE_LIMITED_OPS' is set.

`RE_CHAR_CLASSES'
     If this bit is set, then you can use character classes in lists;
     if this bit isn't set, then you can't.

`RE_CONTEXT_INDEP_ANCHORS'
     If this bit is set, then `^' and `$' are special anywhere outside
     a list; if this bit isn't set, then these characters are special
     only in certain contexts.  *Note Match-beginning-of-line
     Operator::, and See Match-end-of-line Operator.

`RE_CONTEXT_INDEP_OPS'
     If this bit is set, then certain characters are special anywhere
     outside a list; if this bit isn't set, then those characters are
     special only in some contexts and are ordinary elsewhere.
     Specifically, if this bit isn't set then `*', and (if the syntax
     bit `RE_LIMITED_OPS' isn't set) `+' and `?' (or `\+' and `\?',
     depending on the syntax bit `RE_BK_PLUS_QM') represent repetition
     operators only if they're not first in a regular expression or
     just after an open-group or alternation operator.  The same holds
     for `{' (or `\{', depending on the syntax bit `RE_NO_BK_BRACES') if
     it is the beginning of a valid interval and the syntax bit
     `RE_INTERVALS' is set.

`RE_CONTEXT_INVALID_OPS'
     If this bit is set, then repetition and alternation operators
     can't be in certain positions within a regular expression.
     Specifically, the regular expression is invalid if it has:

        * a repetition operator first in the regular expression or just
          after a match-beginning-of-line, open-group, or alternation
          operator; or

        * an alternation operator first or last in the regular
          expression, just before a match-end-of-line operator, or just
          after an alternation or open-group operator.

     If this bit isn't set, then you can put the characters
     representing the repetition and alternation characters anywhere in
     a regular expression.  Whether or not they will in fact be
     operators in certain positions depends on other syntax bits.

`RE_DOT_NEWLINE'
     If this bit is set, then the match-any-character operator matches
     a newline; if this bit isn't set, then it doesn't.

`RE_DOT_NOT_NULL'
     If this bit is set, then the match-any-character operator doesn't
     match a null character; if this bit isn't set, then it does.

`RE_INTERVALS'
     If this bit is set, then Regex recognizes interval operators; if
     this bit isn't set, then it doesn't.

`RE_LIMITED_OPS'
     If this bit is set, then Regex doesn't recognize the
     match-one-or-more, match-zero-or-one or alternation operators; if
     this bit isn't set, then it does.

`RE_NEWLINE_ALT'
     If this bit is set, then newline represents the alternation
     operator; if this bit isn't set, then newline is ordinary.

`RE_NO_BK_BRACES'
     If this bit is set, then `{' represents the open-interval operator
     and `}' represents the close-interval operator; if this bit isn't
     set, then `\{' represents the open-interval operator and `\}'
     represents the close-interval operator.  This bit is relevant only
     if `RE_INTERVALS' is set.

`RE_NO_BK_PARENS'
     If this bit is set, then `(' represents the open-group operator and
     `)' represents the close-group operator; if this bit isn't set,
     then `\(' represents the open-group operator and `\)' represents
     the close-group operator.

`RE_NO_BK_REFS'
     If this bit is set, then Regex doesn't recognize `\'DIGIT as the
     back reference operator; if this bit isn't set, then it does.

`RE_NO_BK_VBAR'
     If this bit is set, then `|' represents the alternation operator;
     if this bit isn't set, then `\|' represents the alternation
     operator.  This bit is irrelevant if `RE_LIMITED_OPS' is set.

`RE_NO_EMPTY_RANGES'
     If this bit is set, then a regular expression with a range whose
     ending point collates lower than its starting point is invalid; if
     this bit isn't set, then Regex considers such a range to be empty.

`RE_UNMATCHED_RIGHT_PAREN_ORD'
     If this bit is set and the regular expression has no matching
     open-group operator, then Regex considers what would otherwise be
     a close-group operator (based on how `RE_NO_BK_PARENS' is set) to
     match `)'.