A Compile time PCRE (almost) compatible regular expression matcher.
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Hana Dusíková
Hana Dusíková Issue #3:
Rename testing namespace RegExp to AltRegExp so there is no name disambiguation.
Uninitialized variable in math.cpp.
Latest commit 9d6db33 Oct 29, 2017


Compile time regular expressions

This library is highly experimental and API will change in future.

It's a library which provides a regular expression parsing during compile time and also compile time evaluation of them.

In case of an error in your pattern it gives you simple static_assert failure and not a long and crazy error somewhere in a templated code.

Internally it using LL(1) parser implemented as C++ templates to transform a string pattern into type. This type can be evaluated during compile time or runtime. Matching pattern is implemented (for now) as depth search with all of it's caveats.


Usually a regular expressions in your code doesn't change and you want them to be quick as possible. You can write finite state automata but they are hard to maintain and do them correctly. An ordinary regular expressions are usually complex libraries which can be used in embeded environment.

Any regular expression is a just common known pattern and they are 1:1 mapped to FA. And the FA can be transformed into a code. So there is no reason for using "interpreted" regular expression.

Future work

  • Support full constexpr-ness with captures and repeat patterns (soon)
  • Support for structured binding of captures:
if (auto [matched, a, b] = "^([a-z]+) ([0-9]+)$"_pre.match(input); matched) {
	// do something with a,b
  • Clean source code
  • Documentation
  • Get it into standard 😄


You can try it on the Matt Godbolt's Compiler Explorer, just enable CTRE library and include <ctre> header.

Transforming string into type

With P0424 proposal (supported by clang and gcc) for C++ usage is very nice and easy:

template<typename CharT, CharT ... string> constexpr auto operator""_fpre() {
	static_assert(std::is_same<CharT, char>::value);
	using Parser = Static::Decider<Stack<YourStartNonTerminal>, Static::Input<string...>>;
	static_assert(Parser::correct, "Can't parse input pattern."); 
	return typename Parser::template Type<>{};

You need define your nonterminals, start symbol (YourStartNonTerminal), alphabet and parser table, for example: math.cpp.

Using regular expression as UDL

#include "pregexp.hpp"

int main(int argc, char ** argv) {
using namespace sre;

// it is really just a type representation of the pattern

// let's try it
if (argc >= 2 && "^(?:[abc]|xyz).+$"_pre.match(argv[1])) {
	return 0;
} else {
	puts("not match");
	return 1;

Output ASM

0000000100000e70	pushq	%rbp
0000000100000e71	movq	%rsp, %rbp
0000000100000e74	cmpl	$0x2, %edi
0000000100000e77	jl	0x100000f3d
0000000100000e7d	movq	0x8(%rsi), %r9
0000000100000e81	movb	(%r9), %al
0000000100000e84	testb	%al, %al
0000000100000e86	je	0x100000f3d
0000000100000e8c	movzbl	%al, %r8d
0000000100000e90	cmpl	$0x61, %r8d
0000000100000e94	je	0x100000ea0
0000000100000e96	andb	$-0x2, %al
0000000100000e98	movzbl	%al, %eax
0000000100000e9b	cmpl	$0x62, %eax
0000000100000e9e	jne	0x100000edb
0000000100000ea0	leaq	0x1(%r9), %rsi
0000000100000ea4	xorl	%edx, %edx
0000000100000ea6	movq	%r9, %rdi
0000000100000ea9	xorl	%r10d, %r10d
0000000100000eac	jmp	0x100000ebf
0000000100000eae	nop
0000000100000eb0	movq	%rdi, %rax
0000000100000eb3	addq	$0x2, %rax
0000000100000eb7	decl	%edx
0000000100000eb9	movq	%rsi, %rdi
0000000100000ebc	movq	%rax, %rsi
0000000100000ebf	testl	%edx, %edx
0000000100000ec1	movb	(%rsi), %al
0000000100000ec3	je	0x100000ed1
0000000100000ec5	testb	%al, %al
0000000100000ec7	movb	$0x1, %cl
0000000100000ec9	je	0x100000ece
0000000100000ecb	movb	%r10b, %cl
0000000100000ece	movb	%cl, %r10b
0000000100000ed1	testb	%al, %al
0000000100000ed3	jne	0x100000eb0
0000000100000ed5	testb	$0x1, %r10b
0000000100000ed9	jne	0x100000f50
0000000100000edb	cmpl	$0x78, %r8d
0000000100000edf	jne	0x100000f3d
0000000100000ee1	movzbl	0x1(%r9), %eax
0000000100000ee6	cmpl	$0x79, %eax
0000000100000ee9	jne	0x100000f3d
0000000100000eeb	movzbl	0x2(%r9), %eax
0000000100000ef0	cmpl	$0x7a, %eax
0000000100000ef3	jne	0x100000f3d
0000000100000ef5	leaq	0x1(%r9), %rcx
0000000100000ef9	addq	$0x2, %r9
0000000100000efd	xorl	%edx, %edx
0000000100000eff	xorl	%esi, %esi
0000000100000f01	nopw	%cs:(%rax,%rax)
0000000100000f10	movq	%r9, %rdi
0000000100000f13	testl	%edx, %edx
0000000100000f15	movb	0x2(%rcx), %al
0000000100000f18	je	0x100000f27
0000000100000f1a	testb	%al, %al
0000000100000f1c	movb	$0x1, %r8b
0000000100000f1f	je	0x100000f24
0000000100000f21	movb	%sil, %r8b
0000000100000f24	movb	%r8b, %sil
0000000100000f27	addq	$0x2, %rcx
0000000100000f2b	decl	%edx
0000000100000f2d	testb	%al, %al
0000000100000f2f	movq	%rcx, %r9
0000000100000f32	movq	%rdi, %rcx
0000000100000f35	jne	0x100000f10
0000000100000f37	testb	$0x1, %sil
0000000100000f3b	jne	0x100000f50
0000000100000f3d	leaq	0x44(%rip), %rdi        ## literal pool for: "not match"
0000000100000f44	callq	0x100000f60             ## symbol stub for: _puts
0000000100000f49	movl	$0x1, %eax
0000000100000f4e	popq	%rbp
0000000100000f4f	retq
0000000100000f50	leaq	0x2b(%rip), %rdi        ## literal pool for: "match"
0000000100000f57	callq	0x100000f60             ## symbol stub for: _puts
0000000100000f5c	xorl	%eax, %eax
0000000100000f5e	popq	%rbp
0000000100000f5f	retq