Add gist on using rx inside lisp-data

gists.org

@@ -1140,6 +1140,50 @@ Which covers my (potential) GPG/SSH usage within Emacs. Now, anytime a I perform
     (run-with-timer nil (* 2 60 60) (lambda () (run-with-idle-timer 120 nil 'elfeed-update))))
 #+end_src
 
+* Use rx style regular expressions in Lisp data files
+
+#+begin_src org :noweb no :tangle gists/rx-in-lisp-data.org
+  Plain regular expressions are difficult to write and sometimes difficult to read, therefore I prefer constructing regular expressions with the =rx= macro. It will take care of the special characters and the proper escaping, as I can't remember those details that well. With =rx=, you construct regular expressions in a Lisp format, e.g.:
+
+  ,#+begin_src elisp
+    (rx string-start (or "foo" "bar") string-end)
+  ,#+end_src
+
+  ,#+RESULTS:
+  : \`\(?:bar\|foo\)\'
+
+  It's not only more readable (in my opinion), but it's also easier to construct with all the Lisp writing tools that can be used within Emacs (e.g. paredit or puni).
+
+  However, using =rx= is not always possible. Lisp data files may contain regular expressions but they are treated as data and not as executable code.
+
+  One prime example is elfeed-score: an Emacs package that allows you to write scoring rules for elfeed entries. Scoring rules are defined with an Lisp data file, and a rule may allow regular expressions but they have to be written out verbatim.
+
+  Luckily there's a work-around with Org Mode:
+
+  - Write the scoring rules in an =lisp-data= source block.
+  - Tangle the file to a location that is read by the elfeed-score package (see variable ~elfeed-score-score-file~).
+  - For the regular expression we use a small =elisp= block as follows:
+
+    ,#+begin_src org
+    ,,#+name: rx
+    ,,#+begin_src elisp :var expression='any :tangle=no
+      (string-replace "\\" "\\\\" (format "\"%s\"" (rx-to-string expression)))
+    ,,#+end_src
+    ,#+end_src
+
+    It accepts an /expression/ that is a Lisp form that =rx= accepts. It is converted to a string, surrounded by quotes and since we use it in a Lisp string, backslashes should be doubled.
+
+  - By naming the source block =rx= we can call it from other source blocks:
+
+    ,#+begin_src org
+    ,,#+begin_src lisp-data :tangle elfeed-scores.eld :noweb yes
+      (("title" (:text <<rx '(or "foo" "bar")>> :value 1 :type r)))
+    ,,#+end_src
+    ,#+end_src
+
+    Upon tangling, the <<rx ...>> is converted to a regex string and therefore readable by elfeed-score.
+#+end_src
+
 * Meta
 ** License
 

gists/rx-in-lisp-data.org

@@ -0,0 +1,39 @@
+Plain regular expressions are difficult to write and sometimes difficult to read, therefore I prefer constructing regular expressions with the =rx= macro. It will take care of the special characters and the proper escaping, as I can't remember those details that well. With =rx=, you construct regular expressions in a Lisp format, e.g.:
+
+#+begin_src elisp
+  (rx string-start (or "foo" "bar") string-end)
+#+end_src
+
+#+RESULTS:
+: \`\(?:bar\|foo\)\'
+
+It's not only more readable (in my opinion), but it's also easier to construct with all the Lisp writing tools that can be used within Emacs (e.g. paredit or puni).
+
+However, using =rx= is not always possible. Lisp data files may contain regular expressions but they are treated as data and not as executable code.
+
+One prime example is elfeed-score: an Emacs package that allows you to write scoring rules for elfeed entries. Scoring rules are defined with an Lisp data file, and a rule may allow regular expressions but they have to be written out verbatim.
+
+Luckily there's a work-around with Org Mode:
+
+- Write the scoring rules in an =lisp-data= source block.
+- Tangle the file to a location that is read by the elfeed-score package (see variable ~elfeed-score-score-file~).
+- For the regular expression we use a small =elisp= block as follows:
+
+  #+begin_src org
+  ,#+name: rx
+  ,#+begin_src elisp :var expression='any :tangle=no
+    (string-replace "\\" "\\\\" (format "\"%s\"" (rx-to-string expression)))
+  ,#+end_src
+  #+end_src
+
+  It accepts an /expression/ that is a Lisp form that =rx= accepts. It is converted to a string, surrounded by quotes and since we use it in a Lisp string, backslashes should be doubled.
+
+- By naming the source block =rx= we can call it from other source blocks:
+
+  #+begin_src org
+  ,#+begin_src lisp-data :tangle elfeed-scores.eld :noweb yes
+    (("title" (:text <<rx '(or "foo" "bar")>> :value 1 :type r)))
+  ,#+end_src
+  #+end_src
+
+  Upon tangling, the <<rx ...>> is converted to a regex string and therefore readable by elfeed-score.