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= HIGHLIGHT REGULAR EXPRESSIONS MANUAL
André Simon
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:GNU-regex-library: pass:[https://www.gnu.org/software/regex/[GNU regular expression library^]]

This file is based on the original Boost API documentation:

* http://www.boost.org/doc/libs/1_32_0/libs/regex/doc/syntax.html

Regular expressions can be applied in highlight's syntax definitions.

== Literals

All characters are literals except: `.`, `|`, `*`, `?`, `+`, `(`, `)`, `{`, `}`,
`[`, `]`, `^`, `$` and `\`.
These characters are literals when preceded by a `\`.
A literal is a character that matches itself, or matches the result of
`traits_type::translate()`, where `traits_type` is the traits template parameter to
class `basic_regex`.

== Wildcard

The dot character `.` matches any single character except: when
`match_not_dot_null` is passed to the matching algorithms, the dot does not
match a null character; when `match_not_dot_newline` is passed to the matching
algorithms, then the dot does not match a newline character.

== Repeats

A repeat is an expression that is repeated an arbitrary number of times.

An expression followed by `*` can be repeated any number of times including zero.

An expression followed by `+` can be repeated any number of times, but at least
once, if the expression is compiled with the flag `regex_constants::bk_plus_qm`
then `+` is an ordinary character and `\+` represents a repeat of once or more.

An expression followed by `?` may be repeated zero or one times only, if the
expression is compiled with the flag `regex_constants::bk_plus_qm` then `?` is an
ordinary character and `\?` represents the repeat zero or once operator.

When it is necessary to specify the minimum and maximum number of repeats explicitly,
the bounds operator `{}` may be used, thus `a{2}` is the letter `a` repeated
exactly twice, `a{2,4}` represents the letter `a` repeated between 2 and 4
times, and `a{2,}` represents the letter `a` repeated at least twice with no
upper limit.

Note that there must be no white-space inside the `{}`, and there is
no upper limit on the values of the lower and upper bounds.

When the expression is compiled with the flag `regex_constants::bk_braces` then
`{` and `}` are ordinary characters and `\{` and `\}` are used to delimit bounds
instead.

All repeat expressions refer to the shortest possible previous sub-expression: a
single character; a character set, or a sub-expression grouped with `()` for
example.

[discrete]
=== Examples

[horizontal]
`ba*`       ::  will match all of "`b`", "`ba`", "`baaa`" etc.
`ba+`       ::  will match "`ba`" or "`baaaa`" for example but not "`b`".
`ba?`       ::  will match "`b`" or "`ba`".
`ba{2,4}`   ::  will match "`baa`", "`baaa`" and "`baaaa`".

== Non-greedy repeats

Whenever the `extended` regular expression syntax is in use (the default) then
non-greedy repeats are possible by appending a `?` after the repeat; a
non-greedy repeat is one which will match the shortest possible string.

For example to match html tag pairs one could use something like:

......................................
<\s*tagname[^>]*>(.*?)<\s*/tagname\s*>
......................................

In this case `$1` will contain the text between the tag pairs, and will be the
shortest possible matching string.

== Parenthesis

Parentheses serve two purposes, to group items together into a sub-expression,
and to mark what generated the match.
For example the expression `(ab)*` would match all of the string "`ababab`".

The matching algorithms `regex_match` and `regex_search` each take an instance
of `match_results` that reports what caused the match, on exit from these
functions the `match_results` contains information both on what the whole
expression matched and on what each sub-expression matched.

In the example above `match_results[1]` would contain a pair of iterators
denoting the final "`ab`" of the matching string.

It is permissible for sub-expressions to match null strings.

If a sub-expression takes no part in a match -- for example if it is part of an
alternative that is not taken -- then both of the iterators that are returned for
that sub-expression point to the end of the input string, and the matched
parameter for that sub-expression is false.

Sub-expressions are indexed from left to right starting from 1, sub-expression 0
is the whole expression.

== Non-Marking Parenthesis

Sometimes you need to group sub-expressions with parenthesis, but don't want the
parenthesis to spit out another marked sub-expression, in this case a
non-marking parenthesis `(?:expression)` can be used.

For example the following expression creates no sub-expressions:

................................................................................
(?:abc)*
................................................................................

== Forward Lookahead Asserts

There are two forms of these; one for positive forward lookahead asserts, and
one for negative lookahead asserts:

[horizontal]
`(?=abc)` :: matches zero characters only if they are followed by the expression "`abc`".
`(?!abc)` :: matches zero characters only if they are not followed by the expression "`abc`".

== Independent sub-expressions

`(?>expression)` matches `expression` as an independent atom (the algorithm will
not backtrack into it if a failure occurs later in the expression).

== Alternatives

Alternatives occur when the expression can match either one sub-expression or
another, each alternative is separated by a `|`, or a `\|` if the flag
`regex_constants::bk_vbar` is set, or by a newline character if the flag
`regex_constants::newline_alt` is set.

Each alternative is the largest possible previous sub-expression; this is the
opposite behavior from repetition operators.

[discrete]
=== Examples:

[horizontal]
`a(b|c)`  :: could match "`ab`" or "`ac`".
`abc|def` :: could match "`abc`" or "`def`".

== Sets

A set is a set of characters that can match any single character that is a
member of the set.
Sets are delimited by `[` and `]` and can contain literals,
character ranges, character classes, collating elements and equivalence classes.
Set declarations that start with `^` contain the complement of the elements that
follow.

[discrete]
=== Examples:

Character literals:

[horizontal]
`[abc]`    :: will match either of "`a`", "`b`", or "`c`".
`[^abc]`   :: will match any character other than "`a`", "`b`", or "`c`".

Character ranges:

[horizontal]
`[a-z]`    :: will match any character in the range "`a`" to "`z`".
`[^A-Z]`   :: will match any character other than those in the range "`A`" to "`Z`".

Note that character ranges are highly locale dependent if the flag
`regex_constants::collate` is set: they match any character that collates between
the endpoints of the range, ranges will only behave according to ASCII rules
when the default "`C`" locale is in effect.

For example if the library is compiled with the Win32 localization model, then
`[a-z]` will match the ASCII characters a-z, and also '`A`', '`B`' etc, but not
'`Z`' which collates just after '`z`'.

This locale specific behavior is disabled by default (in perl mode), and forces ranges to collate according to ASCII character code.

Character classes are denoted using the syntax `[:classname:]` within a set
declaration, for example `[[:space:]]` is the set of all whitespace characters.
Character classes are only available if the flag `regex_constants::char_classes`
is set.

The available character classes are:

[cols="10m,90d",options="header"]
|===============================================================================
| class  | description

There are some shortcuts that can be used in place of the character classes,
provided the flag `regex_constants::escape_in_lists` is set then you can use:

[horizontal]
`\w`  :: in place of `[:word:]`
`\s`  :: in place of `[:space:]`
`\d`  :: in place of `[:digit:]`
`\l`  :: in place of `[:lower:]`
`\u`  :: in place of `[:upper:]`

Collating elements take the general form `[.tagname.]` inside a set declaration,
where _tagname_ is either a single character, or a name of a collating element,
for example `[[.a.]]` is equivalent to `[a]`, and `[[.comma.]]` is equivalent to `[,]`.

The library supports all the standard POSIX collating element names, and in
addition the following digraphs: `ae`, `ch`, `ll`, `ss`, `nj`, `dz`, `lj`, each
in lower, upper and title case variations.

Multi-character collating elements can result in the set matching more than one
character, for example `[[.ae.]]` would match two characters, but note that
`[^[.ae.]]` would only match one character.

Equivalence classes take the general form `[=tagname=]` inside a set declaration,
where _tagname_ is either a single character, or a name of a collating element,
and matches any character that is a member of the same primary equivalence class
as the collating element `[.tagname.]`.

An equivalence class is a set of characters that collate the same, a primary
equivalence class is a set of characters whose primary sort key are all the same
(for example strings are typically collated by character, then by accent, and
then by case; the primary sort key then relates to the character, the secondary
to the accentation, and the tertiary to the case).

If there is no equivalence class corresponding to _tagname_, then `[=tagname=]` is
exactly the same as `[.tagname.]`.

Unfortunately there is no locale independent method of obtaining the primary
sort key for a character, except under Win32.

For other operating systems the library will "`guess`" the primary sort key from
the full sort key (obtained from `strxfrm`), so equivalence classes are probably
best considered broken under any operating system other than Win32.

To include a literal `-` in a set declaration then: make it the first character
after the opening `[` or `[^`, the endpoint of a range, a collating element, or
if the flag `regex_constants::escape_in_lists` is set then precede with an escape
character as in `[\-]`.

To include a literal `[` or `]` or `^` in a set then make them the endpoint of a
range, a collating element, or precede with an escape character if the flag
`regex_constants::escape_in_lists` is set.

== Line anchors

An anchor is something that matches the null string at the start or end of a
line: `^` matches the null string at the start of a line, `$` matches the null
string at the end of a line.

== Back references

A back reference is a reference to a previous sub-expression that has already
been matched, the reference is to what the sub-expression matched, not to the
expression itself.

A back reference consists of the escape character `\` followed by a digit `1` to
`9`, `\1` refers to the first sub-expression, `\2` to the second etc.

For example the expression `(.*)\1` matches any string that is repeated about
its mid-point for example "`abcabc`" or "`xyzxyz`".

A back reference to a sub-expression that did not participate in any match,
matches the null string: NB this is different to some other regular expression
matchers.

Back references are only available if the expression is compiled with the flag
`regex_constants::bk_refs` set.

== Word operators

The following operators are provided for compatibility with the {GNU-regex-library}.

[horizontal]
`\w` :: matches any single character that is a member of the `word` character
        class, this is identical to the expression `[[:word:]]`.
`\W` :: matches any single character that is not a member of the `word` character
        class, this is identical to the expression `[^[:word:]]`.
`\<` :: matches the null string at the start of a word.
`\>` :: matches the null string at the end of the word.
`\b` :: matches the null string at either the start or the end of a word.
`\B` :: matches a null string within a word.

The start of the sequence passed to the matching algorithms is considered to be
a potential start of a word unless the flag `match_not_bow` is set.

The end of the sequence passed to the matching algorithms is considered to be a
potential end of a word unless the flag `match_not_eow` is set.

== Buffer operators

The following operators are provided for compatibility with the {GNU-regex-library},
and Perl regular expressions:

[horizontal]
`\`` :: matches the start of a buffer.
`\A` :: matches the start of the buffer.
`\'` :: matches the end of a buffer.
`\z` :: matches the end of a buffer.
`\Z` :: matches the end of a buffer, or possibly one or more new line characters
        followed by the end of the buffer.

A buffer is considered to consist of the whole sequence passed to the matching
algorithms, unless the flags `match_not_bob` or `match_not_eob` are set.

== Escape operator

The escape character `\` has several meanings.

Inside a set declaration the escape character is a normal character unless the
flag `regex_constants::escape_in_lists` is set in which case whatever follows the
escape is a literal character regardless of its normal meaning.

The escape operator may introduce an operator for example: back references, or a
word operator.

The escape operator may make the following character normal, for example `\*`
represents a literal `*` rather than the repeat operator.

// @ARRIVED HERE

== Single character escape sequences

The following escape sequences are aliases for single characters:

[cols="2*10m,80d",options="header"]
|===============================================================================
| Escape | Char  | Meaning
| \a     | 0x07  | Bell character.
| \f     | 0x0C  | Form feed.
| \n     | 0x0A  | Newline character.
| \r     | 0x0D  | Carriage return.
| \t     | 0x09  | Tab character.
| \v     | 0x0B  | Vertical tab.
| \e     | 0x1B  | ASCII Escape character.
| \0dd   | 0dd   | Bell character.
| \xXX   | 0xXX  | A hexadecimal character code, where _XX_ is one or more
                   hexadecimal digits
| \x{XX} | 0xXX  | A hexadecimal character code, where _XX_ is one or more
                   hexadecimal digits, optionally a Unicode character.
| \cZ    | z-@   | An ASCII escape sequence control-Z, where _Z_ is any ASCII
                   character greater than or equal to the code for `@`.
|===============================================================================

== Miscellaneous escape sequences:

The following are provided mostly for perl compatibility, but note that there
are some differences in the meanings of `\l`, `\L`, `\u` and `\U`:

[horizontal]
`\w` :: Equivalent to `[[:word:]]`.
`\W` :: Equivalent to `[^[:word:]]`.
`\s` :: Equivalent to `[[:space:]]`.
`\S` :: Equivalent to `[^[:space:]]`.
`\d` :: Equivalent to `[[:digit:]]`.
`\D` :: Equivalent to `[^[:digit:]]`.
`\l` :: Equivalent to `[[:lower:]]`.
`\L` :: Equivalent to `[^[:lower:]]`.
`\u` :: Equivalent to `[[:upper:]]`.
`\U` :: Equivalent to `[^[:upper:]]`.
`\C` :: Any single character, equivalent to `.`.
`\X` :: Match any Unicode combining character sequence, for example `a\x0301`
        (a letter a with an acute).
`\Q` :: The begin quote operator, everything that follows is treated as
        a literal character until a `\E` end quote operator is found.
`\E` :: The end quote operator, terminates a sequence begun with `\Q`.

[discrete]
=== Examples:

`Regex=[[ [A-Z]\w+ ]]` +
Highlight identifiers beginning with a capital letter.

`Regex=[[ [$@%]\w+ ]]` +
Highlight variables beginning with $, @ or %.

`Regex=[[ \$\{(\w+)\}) ]]` or `Regex=[[ \$\{(\w+)\} ]], Group=1` +
Highlight variable names like "`${name}`".
Only the name is highlighted as keyword.
A sub-expression is used to achieve this effect.
If no sub-expression number
is defined (like in the first example above), the right-most sub match
(highest sub id) is returned.

`Regex=[[ (\w+)\s*\( ]]` +
Highlight method names.
Note that a sub expression is used again.

`Regex=[[STO\xe2\x88\x91]]` +
Unicode characters in a keyword.

`[[\A(?!x)x]]` +
A never matching expression.
Can be used to disable a default syntax element.

'''

Andre Simon

a.simon@mailbox.org

http://www.andre-simon.de/

Git project with Git repository, bug tracker:

* https://gitlab.com/saalen/highlight/

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