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Reference to 12.1 The Regular Expression for sentence-end of Elisp introduction

It illustrate alternatives of sentence-end as

 \\($\\| \\|  \\)
        ^   ^^
       TAB  SPC

and explains:

Two backslashes, ‘\\’, are required before the parentheses and vertical bars: the first backslash quotes the following backslash in Emacs; and the second indicates that the following character, the parenthesis or the vertical bar, is special.

But what does the vertical bar "|" mean?

  • Please don't use tag elisp, except for questions about the nature of the language itself, as opposed to, for example, other Lisp dialects. – Drew Mar 18 at 15:15
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The vertical bar lets you form an alternation, a regular expression which matches any one of several alternatives. Combined with the parentheses, this one matches either the end of the line, or a tab character, or two space characters. See https://www.gnu.org/software/emacs/manual/html_mono/emacs.html#Regexps for details.

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