How to check with regular expressions or another way if some package is used in a LaTeX document?

Question

How to check if a specific package is used by a LaTeX document. For example in the below document (intentionally written in an ugly but technically correct format), the packages fontenc, inputenc, csquotes, babel, libertine, amsmath, biblatex, filecontents, tikz are used but lettrine, cleveref, tocloft are not used (because they are in a comment area).

    \documentclass[ngerman]{article}
    \usepackage[T1]{fontenc, 
    %babel,
    }
    %\usepackage{cleveref}
    \usepackage
[utf8
]
{inputenc}
    \usepackage{babel, csquotes

    tikz, 
               % lettrine
    amsmath,libertine
    }

    \usepackage[backend=biber, style=authortitle]{biblatex} 
    \usepackage
    {
    filecontents,

    %tocloft}
    }


    \begin{document}

    \end{document}

Answer 1

You should look into TeX-active-styles variable. For this document its value is

("url" "ifthen" "logreq" "kvoptions" "keyval" "etoolbox" "biblatex"
 "libertine" "amsopn" "amsbsy" "amstext" "amsmath" "tikz" "csquotes"
 "ngerman" "babel" "" "fontenc" "art10" "article" "latex2e" "z"
 "LATEX")

Then the function you should use is member:

(member "fontenc" TeX-active-styles)

This is the same check you should do for classes (see also this question).

In addition, if you want to check whether a certain option has been passed to package there is LaTeX-provided-package-options-member

(LaTeX-provided-package-options-member "fontenc" "T1")