If test-string is defined like so

(setq test-string "fooXbarXbazXextraYstuff")

then the regexp "^[^XY]+X" matches test-string:

(string-match "^[^XY]+X" test-string)
=> 0 (#o0, #x0, ?\C-@)

...but the same regexp with grouping parentheses added no longer matches:

(string-match "^\([^XY]+X\)" test-string)
=> nil

Simply adorable.

Given the behavior above, it is not surprising that

(replace-regexp-in-string "^\([^XY]+X\)+" "" test-string)

evaluates to the original value of test-string.

What regexp must I put in place of the ??? below

(replace-regexp-in-string ??? "" test-string)

so that the resulting value is the string "extraYstuff"?

(BTW, I'd love to know the official/documented rationale for the behavior shown above, if there is one.)

  • 2
    You need to escape the backslash, when it is inside string literal (as opposed to when you input the regular expression interactively).
    – wvxvw
    Commented Jan 19, 2016 at 16:01
  • 2
    Yes, escaping rules are tricky -- this is why I prefer to use rx in my elisp code. Commented Jan 22, 2016 at 6:10
  • It's very useful when writing regexps to use M-x regexp-builder as it lets you see live how your changes take effect
    – rlazo
    Commented Jan 22, 2016 at 12:35

1 Answer 1


You need to escape the \ on your \(...\) to read \\(...\\):

(string-match "^\\([^XY]+X\\)" "fooXbarXbazXextraYstuff") ; => 0
  • Oh my god... Well, thanks & +1. (I'll accept your answer as soon as ESE lets me.)
    – kjo
    Commented Jan 19, 2016 at 16:03
  • 1
    @kjo: I make this mistake with great regularity.
    – Dan
    Commented Jan 19, 2016 at 16:05
  • 1
    Instead of bothering with backslash escapes yourself, consider using a tool like helm-regexp. As the Wiki page notes, it has several advantages, including instant feedback for your regexp on a buffer and a Helm action to add its string form to your kill ring. Commented Jan 22, 2016 at 6:40

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