I wanted to create a regex to find these dates:
1966/08/20
2023/02/12
In re-builder I was able to build this:
"\([0-9]\{4\}\)/\([0-9]\{2\}\)/\([0-9]\{2\}\)"
In the buffer the dates are picked up and highlighted.
If I copy this exact same string and invoke isearch-forward-regexp Emacs does not find the dates. However, if I copy the actual regex into the buffer isearch-forward-regexp finds that.
Your question is unclear.
If I copy this exact same string and invoke
isearch-forward-regexpEmacs does not find the dates.
What do you mean by that?
Isearch doesn't automatically search for the last string you copied.
If you mean (and this is my best guess) that you typed C-M-sC-y then that's doing a regexp-quote on the copied text to search for it verbatim (but you would see this in the modified regexp shown in the minibuffer).
If it's not that, then tell us specifically what you are doing (give exact keystrokes), what you are expecting, and what's actually happening?
Edit: The comments confirm that the above guess was accurate, so the solution is as follows...
Rather than doing this: C-M-sC-y (which results in the pasted text being passed through regexp-quote)
Instead, do this: C-M-sM-eC-yRET
I.e.: Call isearch-edit-string to edit the search pattern in the minibuffer and then yank the copied text.
re-builder buffer (and copying without the quotes). Then I close re-builder and invoke C-M-s and yank the copied text using C-y, but Regexp I-search finds nothing.
C-y in an isearch is for finding the pasted text verbatim and so it regexp-quotes the text (which you will observe in the search pattern shown in the echo area). Instead of doing that, you need to type M-e to edit the search pattern, and then C-y to paste the regexp (and RET to resume searching).
re-builder set to string syntax rather than read syntax, otherwise you'll be trying to use the read syntax for elisp code interactively, which will fail as soon as backslashes are involved (amongst some other differences). From your question, it looks like you are already doing this, but I mention it just to be sure. If you're unsure about this side of things, emacs.stackexchange.com/a/5577/454 should help to clarify.
re-builderprovides for different syntax (read,string,rx) (see the docs for explanation). Depending on the syntax, the grouping parens and the multiplier braces have to be preceded by either a single (forstringsyntax) or a double (forreadsyntax) backslash to match. I cannot get any matches without them, so I'm not sure how you manage to get matches inre-builder.[0-9]\{4\}/[0-9]\{2\}/[0-9]\{2\}works (i.e. without the capturing group). As tore-builderI havereb-re-syntaxset to'string. The double escapes made no difference.\(... \): plain parens are not special - they just match themselves.