Filter a table based on a string column

I am trying to filter a table in pgfplots as shown below. It does not compile and I guess this is because it is unable to correctly compare the strings. This example is based on the used guide. For information, I am using luatex, so I need a solution compatible with lualatex compiler please.

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\begin{document}
% Preamble: \pgfplotsset{width=7cm,compat=1.18}
\begin{tikzpicture}
\begin{axis}[grid=major]
    \addplot+ [
        unbounded coords=discard,
        x filter/.expression={\thisrow{type}=="foo" ? x : nan},
    ] table[trim cells=true] {
        x   y type
        1   2 "foo"
        1.5 2.5 "bar"
        2   3 "foo"
        2.5 3.5 "bar"
        3   4 "foo"
    };
\end{axis}
\end{tikzpicture}

\end{document}

I get the following error:

! Package PGF Math Error: Could not parse input '@@str@@:foo' as a floating poi
nt number, sorry. The unreadable part was near '@@str@@:foo'. (in '"foo"=="foo"
 ? x : nan').

Note that if I replace my code by:

    \addplot+ [
        unbounded coords=discard,
        x filter/.expression={\thisrow{type}==42 ? x : nan},
    ] table[trim cells=true] {
        x   y type
        1   2 42
        1.5 2.5 5
        2   3 42
        2.5 3.5 42
        3   4 12
    };

Then it works as expected, i.e., coords x = 1, 2, and 2.5 are plotted while 1.5 is skipped and 3.5 too (x axis stopped at 2.5).

How to compare strings? I found nothing in the user guide...

You could use a .code type of filter:

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\def\foostring{"foo"}

\begin{document}
% Preamble: \pgfplotsset{width=7cm,compat=1.18}
\begin{tikzpicture}
\begin{axis}[grid=major]
    \addplot+ [
        unbounded coords=discard,
        x filter/.code={
          \edef\tempa{\thisrow{type}}
          \ifx\tempa\foostring
              \def\pgfmathresult{}
          \fi
        },
    ] table[trim cells=true] {
        x   y type
        1   2 "foo"
        1.5 2.5 "bar"
        2   3 "foo"
        2.5 3.5 "bar"
        3   4 "foo"
        4   5 "bar"
    };
\end{axis}
\end{tikzpicture}

\end{document}