SRELL

Features

Header-only and the same class design as std::regex

SRELL is a template library that has an ECMAScript (JavaScript) compatible regular expression engine wrapped into the same class design as std::regex. SRELL can be used in the same way as std::regex (or boost::regex on which std::regex is based) and does not need any installation as it is a header-only template libarary.

Unicode-specific implementation

SRELL has native support for Unicode:

• UTF-8, UTF-16, and UTF-32 strings can be handled without any additional configurations.
• '.' does not match a half of a surrogate pair in UTF-16 strings or does not match a code unit in UTF-8 strings.
• Supplementary Characters can be specified in a character class such as [丈𠀋], and a range also can be specified in a character class such as [\u{1b000}-\u{1b0ff}].
• When the case-insensitive match is performed, even characters having two lowercase letters for one uppercase letter such as Greek Σ (u+03c2 [ς] and u+03c3 [σ]) or having the third case called "titlecase" besides the uppercase and the lowercase such as DŽ (uppercase; DŽ, lowercase; dž and titlecase; Dž) are processed appropriately.

Consideration for ignore-case (icase) search

SRELL has been tuned up not to slow down remarkably when case-insensitive (icase) search is performed.

As std::regex was proposed early for C++0x (now C++11), it is little dependent on C++11's new features. So SRELL should be available with even pre-C++11 compilers as far as they interpret C++ templates accurately. (The oldest compiler on where I confirm SRELL can be used is Visual C++ 2005 in Visual Studio 2005).

Download

• SRELL 2.930 (BSD License) 4 Oct 2021 (Revision History)

• Link to the latest version

How to use

No preparation is required. Place srell*.hpp (the three files of srell.hpp, srell_ucfdata2.hpp, and srell_updata.hpp) somewhere in your PATH and include srell.hpp.

If you have used <regex>, you already know how to use SRELL generally.

//  Example 01:
#include <cstdio>
#include <string>
#include <iostream>
#include "srell.hpp"

int main()
{
    srell::regex e;     //  Regular expression object holder.
    srell::cmatch m;    //  Object which receives results.

    e = "\\d+[^-\\d]+"; //  Compile a regular expression string.
    if (srell::regex_search("1234-5678-90ab-cdef", m, e))
    {
        //  If use printf.
        const std::string s(m[0].first, m[0].second);
            //  The code above can be replaced with one of the following lines.
            //  const std::string s(m[0].str());
            //  const std::string s(m.str(0));
        std::printf("result: %s\n", s.c_str());

        //  If use iostream.
        std::cout << "result: " << m[0] << std::endl;
    }
    return 0;
}
	

As in this example, all classes and algorithms that belong to SRELL have been put within namespace "srell". Except for this point, the usage is basically identical to std::regex.

Please see also readme_en.txt included in the zip archive.

#ifdef __cpp_unicode_characters
  #ifndef SRELL_CPP11_CHAR1632_ENABLED
  #define SRELL_CPP11_CHAR1632_ENABLED  //  Do typedef for char16_t and char32_t.
  #endif
#endif

#ifdef __cpp_initializer_lists
  #include <initializer_list>
  #ifndef SRELL_CPP11_INITIALIZER_LIST_ENABLED
  #define SRELL_CPP11_INITIALIZER_LIST_ENABLED  //  Make it possible to pass initializer_list as an argument.
  #endif
#endif

#ifdef __cpp_rvalue_references
  #ifndef SRELL_CPP11_MOVE_ENABLED
  #define SRELL_CPP11_MOVE_ENABLED  //  Enable move semantics in constructos and operator=().
  #endif
#endif

#ifdef __cpp_char8_t
  #ifdef __cpp_lib_char8_t
  #define SRELL_CPP20_CHAR8_ENABLED 2   //  Both char8_t support and std::u8string support.
  #else
  #define SRELL_CPP20_CHAR8_ENABLED 1   //  Only char8_t support.
  #endif
#endif
		

SRELL's regular expression syntax

SRELL 2 supports the regular expressions defined in ECMAScript 2021 Specification 22.2 RegExp (Regular Expression) Objects (The u flag is assumed to be always specified).

The full list is as follows:

Footnotes

• When a sequence of regular expressions ends with a bare '\', or a combination of a backslash and a character which is not explained in the table above appears, error_escape is thrown. Up to version 2.300, SRELL was interpreting the latter as "representing the following character itself", but since version 2.301 handles it as an error in accordance with the ECMAScript specification.
• When you want to write a digit character immediately after a backreference, there are two solutions to prevent them from being interpreted together: 1) write the digit character using its code point, such as /\1\u0030/, or 2) write the digit character inside a character class, which consists of that character only, such as /\1[0]/. SRELL's pattern compiler translates both types of the expressions into the same internal representation.
• The ECMAScript specification does not define any regular expression for the octal escape sequence like \ooo and \0ooo. See also the ECMAScript Specification.
• Until ECMAScript 2017, there is no single-line mode in ECMAScript's RegExp. So, SRELL 1 did not have such a feature. When what matches every code point is needed, [^] (the complementary set of the empty set) can be used for that purpose.
• The positive lookahead assertion can be used to substitute for the AND operation of character classes being available in Java etc. (for example, /(?=\p{sc=Latin})\p{Ll}/ means any lower case letter of the Latin script). Similarly, the negative lookahead assertion can be used to do the subtraction (for example, /(?!\p{sc=Latin})\p{Ll}/ means any lower case letter that is not of the Latin script).
  Note (Feb 2021): set notation proposal (intersection, subtraction, and nested classes) is under consideration in TC39.

Differences between std::regex and SRELL

SRELL extensions for Unicode support

For Unicode support, SRELL has the following typedefs and extensions which do not exist in <regex>:

Typedef list of three basic classes (basic_regex, match_results, sub_match)

Prefix and inter­pretation of string	Specia­lised with ...	basic_regex (-regex)	match_results (-cmatch) (-smatch)	sub_match (-csub_match) (-ssub_match)	Note
u8- (UTF-8)	char8_t or char	u8regex	u8cmatch u8smatch	u8csub_match u8ssub_match	char8_t is used only when char8_t is supported (detected by checking whether the __cpp_char8_t or SRELL_CPP20_CHAR8_ENABLED macro is defined). Otherwise, these types are just aliases of u8c- types shown below.
u16- (UTF-16)	char16_t	u16regex	u16cmatch u16smatch	u16csub_match u16ssub_match	Defined only when char16_t and char32_t are supported (detected by checking whether the __cpp_unicode_characters or SRELL_CPP11_CHAR1632_ENABLED macro is defined).
u32- (UTF-32)	char32_t	u32regex	u32cmatch u32smatch	u32csub_match u32ssub_match
u8c- (UTF-8)	char	u8cregex	u8ccmatch u8csmatch	u8ccsub_match u8cssub_match
u16w- (UTF-16)	wchar_t	u16wregex	u16wcmatch u16wsmatch	u16wcsub_match u16wssub_match	Defined only when 0xFFFF <= WCHAR_MAX < 0x10FFFF.
u32w- (UTF-32)		u32wregex	u32wcmatch u32wsmatch	u32wcsub_match u32wssub_match	Defined only when WCHAR_MAX >= 0x10FFFF.
u1632w-		u1632wregex	u1632wcmatch u1632wsmatch	u1632wcsub_match u1632wssub_match	When 0xFFFF <= WCHAR_MAX < 0x10FFFF, identical to u16w-. When WCHAR_MAX >= 0x10FFFF, identical to u32w-. Unlike u16w- and u32w-, these u1632w- types are always available on condition that WCHAR_MAX is equal to or greater than 0xFFFF.

The meaning of each prefix is as follows:

• u8: meaning varies depending on whether your compiler supports the char8_t type. It is detected by checking whether the __cpp_char8_t or SRELL_CPP20_CHAR8_ENABLED macro is defined or not:
  • If char8_t supported: handles an array of char8_t or an instance of std::u8string as a UTF-8 string.
  • If char8_t not supported: identical to the "u8c-" prefix. Defined as mere aliases of "u8c-" types shown below.
• u16: handles an array of char16_t or an instance of std::u16string as a UTF-16 string.
• u32: handles an array of char32_t or an instance of std::u32string as a UTF-32 string.

• u8c: handles an array of char or an instance of std::string as a UTF-8 string. (Introduced in SRELL version 2.100. Until version 2.002, the "u8-" prefix was used for this kind of type.)
• u16w: handles an array of wchar_t or an instance of std::wstring as a UTF-16 string. (Defined only when WCHAR_MAX is equal to or more than 0xFFFF and less than 0x10FFFF.)
• u32w: handles an array of wchar_t or an instance of std::wstring as a UTF-32 string. (Defined only when WCHAR_MAX is equal to or more than 0x10FFFF.)
• u1632w: when WCHAR_MAX is equal to or more than 0xFFFF and less than 0x10FFFF, identical to u16w- above. When WCHAR_MAX is equal to or more than 0x10FFFF, identical to u32w- above. Types of this prefix are available in SRELL version 2.930 and later.

Although omitted from the table above, based on these rules, regex_iterator and regex_token_iterator that have u(8c?|16w?|32w?|u1632w) prefixes also have been defined similarly.

Basic use of Unicode support versions is as follows:

srell::u8regex u8re(u8"UTF-8 Regular Expression");
srell::u8cmatch u8cm;
std::printf("%s\n", srell::regex_search(u8"UTF-8 target string", u8cm, u8re) ? "found!" : "not found...");

srell::u16regex u16re(u"UTF-16 Regular Expression");
srell::u16cmatch u16cm;
std::printf("%s\n", srell::regex_search(u"UTF-16 target string", u16cm, u16re) ? "found!" : "not found...");

srell::u32regex u32re(U"UTF-32 Regular Expression");
srell::u32cmatch u32cm;
std::printf("%s\n", srell::regex_search(U"UTF-32 target string", u32cm, u32re) ? "found!" : "not found...");

srell::u1632wregex u1632wre(L"UTF-16 or UTF-32 Regular Expression");
srell::u1632wcmatch u1632wcm;
std::printf("%s\n", srell::regex_search(L"UTF-16 or UTF-32 target string", u1632wcm, u1632wre) ? "found!" : "not found...");

srell::u16wregex u16wre(L"UTF-16 Regular Expression");
srell::u16wcmatch u16wcm;
std::printf("%s\n", srell::regex_search(L"UTF-16 target string", u16wcm, u16wre) ? "found!" : "not found...");
    //  The three lines above and the ones below are mutually exclusive.
    //  If wchar_t is less than 21-bit, the ones above are available;
    //  if equal to or more than, the ones below are available.
srell::u32wregex u32wre(L"UTF-32 Regular Expression");
srell::u32wcmatch u32wcm;
std::printf("%s\n", srell::regex_search(L"UTF-32 target string", u32wcm, u32wre) ? "found!" : "not found...");
			

In compilers prior to C++11, only "u8c-" types and "u16w-" types are available if wchar_t is a type being equal to or more than 16-bit and less than 21-bit, and only "u8c-" types and "u32w-" types are available if wchar_t is a type being equal to or more than 21-bit.
However, even in such environments, "u8c-", "u16-" and "u32-" types are available if such code as below is put before including SRELL:

typedef uint_least16_t char16_t;    //  Do typedef for a type that can have a 16-bit value.
typedef uint_least32_t char32_t;    //  Do typedef for a type that can have a 32-bit value.

namespace std
{
    typedef basic_string<char16_t> u16string;
    typedef basic_string<char32_t> u32string;
}

#define SRELL_CPP11_CHAR1632_ENABLED    //  Make them available manually.
			

Incidentally, handling UTF-8 or UTF-16 is performed by u8regex_traits or u16regex_traits passed to basic_regex as a template argument. By using these classes, for example, it is possible to make a class to handle UTF-16 strings with uint32_t type array, such as basic_regex<uint32_t, u16regex_traits<uint32_t> >.

SRELL extensions for the named capture feature

In SRELL 2, the following member functions have been added to the match_results class for the named capture feature:

difference_type length(const string_type &sub) const;
difference_type position(const string_type &sub) const;
string_type str(const string_type &sub) const;
const_reference operator[](const string_type &sub) const;

//  The following ones are available since SRELL 2.650 and later.
difference_type length(const char_type *sub) const;
difference_type position(const char_type *sub) const;
string_type str(const char_type *sub) const;
const_reference operator[](const char_type *sub) const;
			

Basically, these can be used in the same way as the member functions having the same names in regex. The only difference is that these take the group name string as a parameter, instead of the group number corresnponding to a pair of parentheses.

//  Example.
srell::regex e("-(?<digits>\\d+)-");
srell::cmatch m;

if (srell::regex_search("1234-5678-90ab-cdef", m, e))
{
    const std::string by_number(m.str(1));      //  access by paren's number. a feature of std::regex.
    const std::string by_name(m.str("digits")); //  access by paren's name. an extension of SRELL 2.

    std::printf("results: bynumber=%s byname=%s\n", by_number.c_str(), by_name.c_str());
}
//  results: bynumber=5678 byname=5678
			

SRELL extension to syntax_option_type

The following flag option has been added to SRELL 2:

namespace regex_constants
{
    static const syntax_option_type dotall; //  specified the singleline mode.
        //  If specified, the behaviour of '.' is changed.
        //  This flag option corresponds to //s in Perl 5.
}
			

Like the other values of the syntax_option_type type, this value is also defined also in basic_regex.

SRELL extension to error_type

The following error type value has been added to SRELL 2.630 and later:

namespace regex_constants
{
    static const error_type error_utf8;
        //  Invalid UTF-8 sequence is found in a regular expression passed to basic_regex.
}
			

SRELL extensions to regex_search

In SRELL 2.600 and later, there are overload functions that take three BidirectionalIterator as parameters:

template <class BidirectionalIterator, class Allocator, class charT, class traits>
bool regex_search(
    BidirectionalIterator first,
    BidirectionalIterator last,
    BidirectionalIterator lookbehind_limit,
    match_results<BidirectionalIterator, Allocator> &m,
    const basic_regex<charT, traits> &e,
    const regex_constants::match_flag_type flags = regex_constants::match_default);

template <class BidirectionalIterator, class charT, class traits>
bool regex_search(
    BidirectionalIterator first,
    BidirectionalIterator last,
    BidirectionalIterator lookbehind_limit,
    const basic_regex<charT, traits> &e,
    const regex_constants::match_flag_type flags = regex_constants::match_default);
			

The third iterator, lookbehind_limit is used for specifying the limit until where regex_search() can read a sequence backwards when a lookbehind assertion is performed.

const char text[] = "0123456789abcdefghijklmnopqrstuvwxyz";
const char* const begin = text;
const char* const end = text + std::strlen(text);
const char* const first = text + 10;    //  Sets the position of 'a'.
const srell::regex re("(?<=^\\d+).");
srell::cmatch match;

std::printf("matched %d\n", srell::regex_search(first, end, match, re));
    //  Does not match as lookbehind is performed only in the range [first, end).

std::printf("matched %d\n", srell::regex_search(first, end, begin, match, re));
    //  Matches because regex_search is allowed to lookbehind until begin.
    //  I.e., in a three-iterators version, searching againist the sequence
    //  [begin, end), begins at first in the sequence.
			

As shown in the example above, in a "three-iterators version", ^ matches begin (the third iterator) instead of first (the first iterator).

And when a three-iterators version is called, the position() member of match_results returns a distance from the position passed to as the third iterator, while prefix().first of match_results is set to the position passed to as the first iterator.

* By introducing these three-iterators overloads, the way used in SRELL 2.300~2.500 has been removed.

Regular expression engines and flags

<regex> has six regular expression engines including ECMAScript-based one, whereas SRELL has one engine being compatible with ECMAScript.'s RegExp. Because of this difference, the following flag options defined in <regex> are ignored in SRELL even if specified:

syntax_option_type (and flag_type of basic_regex)

• nosubs, optimize, collate, basic, extended, awk, grep, egrep (all but icase and multiline)

match_flag_type

• match_any, format_sed

The comparison between the ECMAScript mode of <regex> and SRELL is as follows:

• <regex>'s ECMAScript mode consists of the expressions defined in the ECMAScript specificatoin third edition
  - (MINUS) Unicode dependent matters (such as meaning of \s)
  + (PLUS) locale dependent matters
  + (PLUS) [:class name:], [.class name.], [=class name=] expressions.

• SRELL 2 consists of the expressions defined in the ECMAScript 2018 or later specification.
• SRELL 1 consists of the expressions defined in the ECMAScript 2017 (ES8) specification
  + (PLUS) fixed-length lookbehind assertions.

Although both are based on the same ECMAScript's regular expression definition, neither <regex> nor SRELL is a superset of each other.

SRELL with char, wchar_t

Types other than u8regex, u8cregex, u16regex, and u16wregex treat an input string as a sequence of Unicode values. For example, when CHAR_BIT is 8, srell::regex (typedef of srell::basic_regex<char>) interprets 0x00-0xFF in an input string as U+0000-U+00FF, respectively. Because the characters represented by U+0000-U+00FF in Unicode are identical to ISO-8859-1, as a result, it can be assumed that srell::regex supports ISO-8859-1.

srell::regex can be used to find a specific pattern of bytes in a binary data.

This applies also to srell::wregex (typedef of srell::basic_regex<wchar_t>). It interprets an input as a sequence of Unicode values in the range 0x00-WCHAR_MAX.

Simplification

The implementations of the following functions of SRELL have been simplified to avoid redundant overheads:

• basic_regex::assign(): In <regex> when an exception is thrown (when compiling a regular expression string fails) *this remains unchanged (cf. 11 in [re.regex.assign]), whereas *this is cleared in SRELL. This is because when compiling begins, SRELL does not keep the old contents anywhere.
• match_results::operator[](size_type n): While <regex> guarantees safety even when n >= match_results::size() (i.e., out-of-range access) (cf. 8 in [re.results.acc]), SRELL does not. Guaranteeing safety needs an additional dummy member of the sub_match type only for the purpose of preparing out-of-range access.

Measures against long time thinking

The backtracking algorithm used by the regular expression engine of ECMAScript (and also Perl on which it is based) can require exponential time to search when given regular expressions include nested quantifiers or consecutive expressions having a quantifier each, and the character sets of which are not mutually exclusive. The following are well-known examples:

• "aaaaaaaaaaaaaaaaaaaaaaaaaaaaa" =~ /(a*)*b/
• "aaaaaaaaaaaaaaaaaaaaaaaaaaaaa" =~ /a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a?aaaaaaaaaaaaaaaaaaaaaaaaaaaaa/

Unfortunately, no fundamental measures against this problem which can be applied to in any situation have found yet. So, to avoid holding control for long time, SRELL throws regex_error(regex_constants::error_complexity) when matching from a particular position fails repeatedly more than certain times.

The default value of the "certain times" is 16777216 (256 to the third power). But this value can be changed by setting an arbitrary value to the limit_counter member variable of an instance of the regex_basic type passed to an algorithm function, such as regex_search() and regex_match().

Breaking changes

const char text[] = "0123456789abcdefghijklmnopqrstuvwxyz";
const char* const begin = text;
const char* const end = text + std::strlen(text);
const char* const first = text + 10;    //  Sets the position of 'a'.
const srell::regex re("(?<=^\\d+).");
srell::cmatch match;

match.lookbehind_limit = begin;

std::printf("matched %d\n", srell::regex_search(first, end, match, re));
    //  Does not match as lookbehind is performed only in the range [first, end).

std::printf("matched %d\n", srell::regex_search(first, end, match, re, srell::regex_constants::match_lblim_avail));
    //  Matches because regex_search is allowed to lookbehind until match.lookbehind_limit.
    //  I.e., when match_lblim_avail specified, searching againist the sequence
    //  [match.lookbehind_limit, end), begins at first in the sequence.
		

"abcd" =~ /(?<=(.){2})./
//  In the case of fixed-length lookbehind: $& = "c", $1 = "b".
//  As the automaton runs from left to right even in a lookbehind assertion,
//  "b" just before "c" is the last one that $1 captured.

//  In the case of variable-length lookbehind: $& = "c", $1 = "a".
//  As the automaton runs from right to left in a lookbehind assertion,
//  "a" being the second next of "c" is the last one that $1 captured.
		

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