How to ignore keywords in regex before variables?

I trying to create a function which is capable of

• putting precision modulator keywords in front of float variables where there is no such a keyword

So it converts

const float x;

to

lowp const float x;

However I would like to ignore the following scenarios:

• When there is already a precision modifier: lowp const float x;
• When there is a default precision modulator: precision lowp float;

Based on my previous question link,This is my regex:

  (\bhighp((?:\s+\w+)*)(float|(?:i|b)?vec[2-4]|mat[2-4](?:x[2-4])?)(*SKIP)(?!)|\bfloat\b)

So, I just would like to ignore those cases, when there is a highp|lowp|mediump before float.

I have this regex command:

#include <iostream>
#include <string>
#include <boost/regex/v5/regex.hpp>

using namespace boost;

std::string precisionModulation(std::string& shaderSource) {

    const regex highpFloatRegex2(R"(highp|lowp|mediump((?:\s+\w+)*)(float)(*SKIP)(?!)|(?=\s+(float)))");
    shaderSource = regex_replace(shaderSource, highpFloatRegex2, "lowp");

    return shaderSource;
}

int main() {
    std::string shaderSource = R"(
float foo1;
highp const float foo1;
precision highp float foo2;
    )";

    std::cout << "Original Shader Source:\n" << shaderSource << std::endl;
    std::string modifiedSource = precisionModulation(shaderSource);
    std::cout << "\nModified Shader Source:\n" << modifiedSource << std::endl;

    return 0;
}

Unfortunately I got weird results:

Original Shader Source:

float foo1;
highp const float foo1;
const highp float foo1;
precision highp float foo2;


Modified Shader Source:

lowp float foo1;
highp const lowp float foo1;
const highp lowp float foo1;
precision highp lowp float foo2;

I also tried:

\w*(?<!highp)((?:\s+\w+)*)\s+float

I suggest using

\bhighp(?:\s+\w+)*\s+(?:float|[ib]?vec[2-4]|mat[2-4](?:x[2-4])?)(*SKIP)(?!)|\b(?:const\s+)?float\b

See the regex demo.

Details:

• \bhighp(?:\s+\w+)*\s+(?:float|[ib]?vec[2-4]|mat[2-4](?:x[2-4])?)(*SKIP)(?!) - matches and skips the following:
  • \b - word boundary
  • highp - a word highp
  • (?:\s+\w+)* - zero or more occurrences of one or more whitespaces followed with one or more word chars
  • \s+ - one or more whitespaces (in your original pattern, you are missing this pattern and it prevented from matching anything)
  • (?:float|[ib]?vec[2-4]|mat[2-4](?:x[2-4])?) - float, or ivec/bvec/vec followed with 2, 3 or 4, or mat followed with 2, 3 or 4 and then an optional x followed with 2, 3 or 4
• | - or
• \b(?:const\s+)?float\b - a whole word float that is optionally preceded with const and one or more whitespaces.

In your code, it should look like

const regex highpFloatRegex2(R"(\bhighp(?:\s+\w+)*\s+(?:float|[ib]?vec[2-4]|mat[2-4](?:x[2-4])?)(*SKIP)(?!)|\b(?:const\s+)?float\b)");
shaderSource = regex_replace(shaderSource, highpFloatRegex2, "lowp $0");

Note that the $0 in the replacement pattern stands for the whole match value.