It is slow. Constructing a std::regex compiles the pattern and allocates; doing it inside a loop is the single most common performance mistake with this header. Hoist it out, or make it static const. Even then std::regex is markedly slower than the regex engines in other languages, and if it is on a hot path, a hand-written scan or a library such as RE2 or CTRE will beat it by an order of magnitude.

It is not a parser. Regular expressions describe regular languages, and nested structure is not regular. HTML, JSON, C++ and arithmetic expressions all need a parser. A regex that almost works on them is worse than no regex, because it fails on the input you did not think of.

Some patterns explode. A pattern like (a+)+b against a long run of a characters can take time exponential in the input length, because the engine tries every way of splitting the run. If a pattern ever touches text from outside your program, that is a denial-of-service waiting to happen.