Why am I not Able to Construct This Regex

Jonathan Mee Published at Dev

Jonathan Mee

I wrote this regex:

(.+?)\s*{?\s*(.+?;)\s*}?\s*

Which tests fine: https://regex101.com/r/gD2eN7/1

But when I try to construct it in C++ I get a runtime-error.

Unhandled exception at 0x7714C52F in temp2.exe: Microsoft C++ exception:
std::regex_error at memory location 0x003BF2EC.

const auto input = "if (KnR)\n\tfoo();\nif (spaces) {\n    foo();\n}\nif (allman)\n{\n\tfoo();\n}\nif (horstmann)\n{\tfoo();\n}\nif (pico)\n{\tfoo(); }\nif (whitesmiths)\n\t{\n\tfoo();\n\t}"s;

cout << input << endl;

cout << regex_replace(input, regex("(.+?)\\s*{?\\s*(.+?;)\\s*}?\\s*"), "$1 {\n\t$2\n}\n") << endl;

Live Example

Am I using functionality that C++ doesn't support?

Wiktor Stribiżew

You need to escape the curly braces. See the std::regex ECMAScript flavor reference:

\character
The character character as it is, without interpreting its special meaning within a regex expression. Any character can be escaped except those which form any of the special character sequences above.
Needed for: ^ $ \ . * + ? ( ) [ ] { } |

regex_replace(input, regex("(.+?)\\s*\\{?\\s*(.+?;)\\s*\\}?\\s*"), "$1 {\n\t$2\n}\n")

Here is an IDEONE demo

#include <iostream>
#include <regex>
#include <string>

using namespace std;

int main() {
    const auto input = "if (KnR)\n\tfoo();\nif (spaces) {\n    foo();\n}\nif (allman)\n{\n\tfoo();\n}\nif (horstmann)\n{\tfoo();\n}\nif (pico)\n{\tfoo(); }\nif (whitesmiths)\n\t{\n\tfoo();\n\t}"s;

    cout << regex_replace(input, regex("(.+?)\\s*\\{?\\s*(.+?;)\\s*\\}?\\s*"), "$1 {\n\t$2\n}\n") << endl;
    //                                           ^^                ^^
}

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