GNU Emacs Manual - Regexp Replace

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### Regexp Replacement

The `M-x replace-string` command replaces exact matches for a single string. The similar command `M-x replace-regexp` replaces any match for a specified pattern.

In `replace-regexp`, the newstring need not be constant: it can refer to all or part of what is matched by the regexp. ``\&'` in newstring stands for the entire match being replaced. ``\d'` in newstring, where d is a digit, stands for whatever matched the dth parenthesized grouping in regexp. To include a ``\'` in the text to replace with, you must enter ``\\'`. For example,

    M-x replace-regexp RET c[ad]+r RET \&-safe RET

replaces (for example) ``cadr'` with ``cadr-safe'` and ``cddr'` with ``cddr-safe'`.

    M-x replace-regexp RET \(c[ad]+r\)-safe RET \1 RET

performs the inverse transformation.

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