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.
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 3. Common Operators,
4. GNU Operators, and 5. 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 section 3.6 List Operators ([ ... ] and [^ ... ])
quotes (makes ordinary, if it's special) the following character; if
this bit isn't set, then `\' is an ordinary character inside lists.
(See section 2.4 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.
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 `)'.
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