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||
Combined from primary sources listed below.

In Regexes§

See primary documentation in context for Alternation: ||

To match one of several possible alternatives, separate them by ||; the first matching alternative wins.

For example, ini files have the following form:

[section]
key = value

Hence, if you parse a single line of an ini file, it can be either a section or a key-value pair and the regex would be (to a first approximation):

/ '[' \w+ ']' || \S+ \s* '=' \s* \S* /

That is, either a word surrounded by square brackets, or a string of non-whitespace characters, followed by zero or more spaces, followed by the equals sign =, followed again by optional whitespace, followed by another string of non-whitespace characters.

An empty string as the first branch is ignored, to allow you to format branches consistently. You could have written the previous example as

/
|| '[' \w+ ']'
|| \S+ \s* '=' \s* \S*
/

Even in non-backtracking contexts, the alternation operator || tries all the branches in order until the first one matches.