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Regular Expression Search
A "regular expression" ("regexp", for short) is a pattern that
denotes a class of alternative strings to match, possibly infinitely
many. In GNU Emacs, you can search for the next match for a regexp
either incrementally or not.
Incremental search for a regexp is done by typing `C-M-s'
(`isearch-forward-regexp'). This command reads a search string
incrementally just like `C-s', but it treats the search string as a
regexp rather than looking for an exact match against the text in the
buffer. Each time you add text to the search string, you make the
regexp longer, and the new regexp is searched for. To search backward
in the buffer, use `C-M-r' (`isearch-backward-regexp').
All of the control characters that do special things within an
ordinary incremental search have the same function in incremental regexp
search. Typing `C-s' or `C-r' immediately after starting the search
retrieves the last incremental search regexp used; that is to say,
incremental regexp and non-regexp searches have independent defaults.
They also have separate search rings that you can access with `M-p' and
If you type SPC in incremental regexp search, it matches any
sequence of whitespace characters, including newlines. If you want to
match just a space, type `C-q SPC'.
Note that adding characters to the regexp in an incremental regexp
search can make the cursor move back and start again. For example, if
you have searched for `foo' and you add `\|bar', the cursor backs up in
case the first `bar' precedes the first `foo'.
Nonincremental search for a regexp is done by the functions
`re-search-forward' and `re-search-backward'. You can invoke these
with `M-x', or bind them to keys, or invoke them by way of incremental
regexp search with `C-M-s RET' and `C-M-r RET'.
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