Template Regex

Library is experimental and shouldn't be used in production code.

Small regular expression library that compile regular expresions during compile time. Other libraries parse expression string during runtime and create internal structures that are used to match input string. This library parses expression string during compile time and create code that is almost same as hand coded code for matching. This solution allows to overperform other libraries but requires to know the regular expression during compile time.

The library lacks support for matching results and can be used only checking if string match given expression.

I've created rules part of this library as replacement for matching code (hand written) in my tokenizers where were same code patterns. After some testing I've realized the rules are de facto regular expressions and this offers me an oportunity to white code that converts regular expression strings to those rules.

The most of code have origin in 2012 and 2013 but I haven't time to finish and release it.

Parts

The whole library are split into 3 main parts.

Template String

This part handle storing string into C++ type.

auto str = string<'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!'>{};

There is a complex macro that create template string type from string literal.

auto str = make_string("Hello World!");

Rules

Rules contains most of the library functionality. Contains multiple classes that represent regular expression rules for matching.

val<'a'>                // Matches 'a' character
range<'a', 'z'>         // Matches character from range 'a' - 'z'
sequence<Rules...>      // Matches sequence of rules.
repeat<Rule>            // Matches one or more occurrence of inner rule.
repeat_optional<Rule>   // Matches zero or more occurrence of inner rule.
optional<Rule>          // Matches optional occurence of inner rule.

Mixing those types into one type it's possible to express any matching rule.

Template Regex

Part only translate regular expression string (stored in template string) into rules.

using namespace template_regex;
auto regex = make_regex("ab+c*");

The regex variable has type that represents given regular expression string:

using namespace template_regex::rules;
sequence<val<'a'>, repeat<val<'b'>>, repeat_optional<val<'c'>>>

using namespace template_regex;
std::string input = "abbc";
regex_match(regex, input);

Performance

Because the library generate code during compile time that allows to optimize source code and doesn't require runtime data to work, library overperform other regular expression libraries.

This table shows how different regex libraries perform in simple matching test of 1 000 000 strings read from file (randomly generated). Time is measured without data loading and regular expression compiling. Source code of those tests is available in 'perf' project.

Library	Identifier	Date	Float
Boost Regex	221 ms	270 ms	334 ms
Boost Xpression	526 ms	579 ms	561 ms
POSIX Regex	235 ms	91 ms	163 ms
Standard Library Regex	284 ms	211 ms	292 ms
Java Regex	348 ms	164 ms	192 ms
Handwritten	41 ms	9 ms	11 ms
Template Regex	30 ms	9 ms	11 ms

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