CTest: more docs on -R regular expression filtering (ECP-WarpX#5257)

* CTest: more docs on `-R` regular expression filtering

* Simplify

* Regex101 Lijnk

* Rephrase

* Rephrase

Co-authored-by: Edoardo Zoni <59625522+EZoni@users.noreply.github.com>

---------

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

Docs/source/developers/testing.rst

@@ -81,6 +81,18 @@ For easier debugging, it can be convenient to run the tests on your local machin
 
        ctest --test-dir build -E laser_acceleration
 
+* Sometimes two or more tests share a large number of input parameters and differ by a small set of options.
+  Such tests typically also share a base string in their names.
+  For example, you can find three different tests named ``test_3d_langmuir_multi``, ``test_3d_langmuir_multi_nodal`` and ``test_3d_langmuir_multi_picmi``.
+  In such a case, if you wish to run the test ``test_3d_langmuir_multi`` only, this can be done again with the ``-R`` regular `expression filter <https://regex101.com>`__ via
+
+  .. code-block:: sh
+
+       ctest --test-dir build -R "test_3d_langmuir_multi\..*"
+
+  Note that filtering with ``-R "test_3d_langmuir_multi"`` would include the additional tests that have the same substring in their name and would not be sufficient to isolate a single test.
+  Note also that the escaping ``\.`` in the regular expression is necessary in order to take into account the fact that each test is automatically appended with the strings ``.run``, ``.analysis`` and possibly ``.cleanup``.
+
 * Run only tests not labeled with the ``slow`` label:
 
   .. code-block:: sh
@@ -161,7 +173,7 @@ A new test can be added by adding a corresponding entry in ``CMakeLists.txt`` as
 
 If you need a new Python package dependency for testing, please add it in `Regression/requirements.txt <https://github.com/ECP-WarpX/WarpX/blob/development/Regression/requirements.txt>`__.
 
-Sometimes, two tests share a large number of input parameters. The shared input parameters can be collected in a "base" input file that can be passed as a runtime parameter in the actual test input files through the parameter ``FILE``.
+Sometimes two or more tests share a large number of input parameters. The shared input parameters can be collected in a "base" input file that can be passed as a runtime parameter in the actual test input files through the parameter ``FILE``.
 
 Naming conventions for automated tests
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