Here is a complicated regexp which was formerly used by Emacs to
recognize the end of a sentence together with any whitespace that
follows. (Nowadays Emacs uses a similar but more complex default
regexp constructed by the function
See Standard Regexps.)
First, we show the regexp as a string in Lisp syntax to distinguish spaces from tab characters. The string constant begins and ends with a double-quote. ‘\"’ stands for a double-quote as part of the string, ‘\\’ for a backslash as part of the string, ‘\t’ for a tab and ‘\n’ for a newline.
"[.?!]\"')}]*\\($\\| $\\|\t\\| \\)[ \t\n]*"
In contrast, if you evaluate this string, you will see the following:
"[.?!]\"')}]*\\($\\| $\\|\t\\| \\)[ \t\n]*" ⇒ "[.?!]\"')}]*\\($\\| $\\| \\| \\)[ ]*"
In this output, tab and newline appear as themselves.
This regular expression contains four parts in succession and can be deciphered as follows:
\"is Lisp syntax for a double-quote in a string. The ‘*’ at the end indicates that the immediately preceding regular expression (a character alternative, in this case) may be repeated zero or more times.
\\($\\| $\\|\t\\| \\)