How to reproduce:
1. Open "RegexBug.zip"
2. In Project Settings -> Player -> Scripting Backend set IL2CPP
3. Build the project

Expected result: Can successfully build the project
Actual result: Exception: Unity.IL2CPP.Building.BuilderFailedException: D:\UnityProjects\case_1288271-RegexBug\Library\Il2cppBuildCache\Android\armeabi-v7a\il2cppOutput\Assembly-CSharp.cpp:697:57: error: use of undeclared identifier 'L_0' Regex__ctor_m8BB2137D7B0A93CD70FD3061579C7456BB65216E(L_0, _stringLiteral894E5AF8C7FAE49021410D34DB96320832C6DDBD, /hidden argument/NULL);

Reproducible with: 2018.4.28f1, 2019.4.13f1, 2020.1.11f1, 2020.2.0a20, 2021.1.0a3
Not reproducible with: 2018.4.27f1, 2019.4.12f1, 2020.1.10f1, 2020.2.0a19

Notes:
- Regex Code that does not allow the project to be built, it is not used anywhere in the scene:
private Regex m_Regex = new Regex(@"[^\\// +-_='.~!?@#$%^&*[\]()a-zA-Z0-9\p{C}\p{So}\u20E3]");
- Does not build for Windows Standalone and Android
- Builds for iOS, but when deploying getting an error regarding REGEX undeclared identifier
- Can build and run on Mac
- Builds on mono