C++ Regex "Parenthesis is not closed." Error

Question

In a game I'm making, I'm using a Regex expression to be able to parse in level data from a file. To test this, I'm trying to use the Regex expression (?<=(LEVEL_TYPE:\s))(\w+|[+-]*\d+) to try and get the level type data in the file which is formatted like LEVEL_TYPE: UNDERWATER

This is my code:

   std::string RegexPattern("(?<=(LEVEL_TYPE:\\s))(\\w+|[+-]*\\d+)");
   std::string target = "LEVEL_TYPE: UNDERWATER";
   std::regex reg(RegexPattern);

Without even using the regex_match method, the program throws an error why I try to run the code. When I try to run it, it throws the error:

terminate called after throwing an instance of 'std::regex_error'
  what():  Parenthesis is not closed.

I'm trying to compile on MinGW on Windows, GCC version 6.3.0

Sometimes parsing a string using a grammar is simpler than using a Regex. — Thomas Matthews, Feb 26 '22 at 00:54
`(?<=` is not valid syntax, see https://en.cppreference.com/w/cpp/regex/ecmascript#Assertions. — user17732522, Feb 26 '22 at 01:00
@user17732522 well if I can't use a positive lookbehind then how am I supposed to be able to do this? I can't think of any possible way to do it. — Gold87, Feb 26 '22 at 01:03
@Gold87 I'll let someone more familiar with regex answer what the correct alternative to the lookbehind is, but aside from that `boost::regex` probably supports it. `std::regex` is known to have problems. — user17732522, Feb 26 '22 at 01:13

score 3 · Accepted Answer · answered Feb 26 '22 at 01:14

3

I can't see a reason for the lookbehind. Isn't this enough?

std::string RegexPattern("(LEVEL_TYPE:\\s)(\\w+|[+-]*\\d+)");

answered Feb 26 '22 at 01:14

zb226

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Or `"LEVEL_TYPE:\\s(\\w+|[+-]*\\d+)"` if you don't want the first part to be captured. – David K Feb 26 '22 at 01:15

C++ Regex "Parenthesis is not closed." Error

1 Answers1