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 current syntax (as set by
re_set_syntax) when the pattern buffer of that regular expression
was compiled.
You get a pattern buffer by compiling a regular expression. See GNU Pattern Buffers, for more information on pattern buffers. See GNU Regular Expression Compiling, and BSD Regular Expression Compiling, for more information on compiling.
Regex considers the current syntax 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 Common Operators and GNU 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 ([ … ] 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 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 ‘$’ are special anywhere outside
a list; if this bit isn’t set, then these characters are special only in
certain contexts. See The Match-beginning-of-line Operator (^), and
The Match-end-of-line Operator ($).
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_DUP ¶If this bit is set, then an open-interval operator cannot occur at the start of a regular expression, or immediately after an alternation, open-group or close-interval operator.
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:
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_DEBUG ¶If this bit is set, and the regex library was compiled with
-DDEBUG, then internal debugging is turned on; if unset, then
it is turned off.
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_HAT_LISTS_NOT_NEWLINE ¶If this bit is set, nonmatching lists ‘[^...]’ do not match newline; if not set, they do.
RE_ICASE ¶If this bit is set, then ignore case when matching; otherwise, case is significant.
RE_INTERVALS ¶If this bit is set, then Regex recognizes interval operators; if this bit isn’t set, then it doesn’t.
RE_INVALID_INTERVAL_ORD ¶If this bit is set, a syntactically invalid interval is treated as a string of ordinary characters. For example, the extended regular expression ‘a{1’ is treated as ‘a\{1’.
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_NO_GNU_OPS ¶If this bit is set, GNU regex operators are not recognized; otherwise, they are.
RE_NO_POSIX_BACKTRACKING ¶If this bit is set, succeed as soon as we match the whole pattern, without further backtracking. This means that a match may not be the leftmost longest; see What Gets Matched? for what this means.
RE_NO_SUB ¶If this bit is set, then no_sub will be set to one during
re_compile_pattern. This causes matching and searching routines
not to record substring match information.
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 ‘)’.