match-data fails to consider only last search with string-match and persists across sessions [duplicate]

Question

Using emacs 25.3.1, I am trying to access the match data after I search a string with string-match. But the match data is wrong. To find out why I tried out the manual's simple example.

    (string-match "\\(qu\\)\\(ick\\)"
                       "The quick fox jumped quickly.")
                       ;0123456789
    (match-string 0 "The quick fox jumped quickly.")

When running this example I get the following results:

4

Debugger entered--Lisp error: (args out of range "The quick fox jumped quickly." 1513 1515)

I think this is because I used string-match on a string before executing this example. However, I expected match-data to only consider my last string-match search and return "quick".

Surprisingly, this behavior persists across sessions. If, after getting this error, I startup emacs with emacs -Q (no init file), and I paste the "quick fox" example and execute it I get the same error (with updated numbers).

I tried to reset match data myself using set-match-data, however that hasn't been working.

(match-data) ; => (1520 1520)
(set-match-data (list 0 0)) ; => nil
(match-data) ; => (1547 1547)

How can I reset the match data myself to produce expected values from string-match search?

Answer 1

The match object you're accessing is global and may be changed by any piece of lisp code. Therefore you should immediately check its state. The following piece of code shows how and exhibits the expected behavior:

(let ((string "The quick fox jumped quickly.")
      (regexp "\\(qu\\)\\(ick\\)"))
  (when (string-match regexp string) ;=> See Note.
    (match-string 0 string))) ;=> "quick"

If you evaluate the lines one by one, the match object will most certainly be mutated in that time, like by syntax highlighting code in Emacs.

*Note: match-string is stateful and "can" persist on consecutive searches even if your next string-match search returns nil.

Answer 2

Actually, you are not passing the same (eq) string to match-data. You are passing a different string that has exactly the same chars. Try this instead:

(setq foo "The quick fox jumped quickly.")

(string-match "\\(qu\\)\\(ick\\)" foo)

(match-string 0 foo)

But besides that, it does work for two different strings that have the same chars. You must have done something in between that changed the match data or something.

Try just this - nothing else:

(string-match "\\(qu\\)\\(ick\\)" "The quick fox jumped quickly.")

(match-string 0 "The quick fox jumped quickly.")

If you still see a problem, provide a complete recipe to reproduce it, starting from emacs -Q (no init file).