SONARJAVA-5098 Fix regexp that finds '\n'

[Ivan-Igorevich]

• 'String#contains()` method finds '\n' as a false-positive new line symbol. In several cases symbols, including 'n', may follow an escaped backslash to generate '\n' as a printed out string, e.g. LaTeX code generator.

• [ ] Unit tests are passing
• [ ] Provided a unit test for your fix
• [ ] ITs should pass : To run ITs locally, checkout the README of the project.