How does {m}{n} (“exactly n times” twice) work?

Question

So, some way or another (playing around), I found myself with a regex like \\d{1}{2}.

Logically, to me, it should mean:

(A digit exac

Answer 1

When I input your regex in RegexBuddy using the Java regex syntax, it displays following message

Quantifiers must be preceded by a token that can be repeated «{2}»

Changing the regex to explicitly use a grouping ^(\d{1}){2} solves that error and works as you expect.

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I assume that the java regex engine simply neglects the error/expression and works with what has been compiled so far.

Edit

The reference to the IEEE-Standard in @piet.t's answer seems to support that assumption.

Edit 2 (kudos to @fncomp)

For completeness, one would typically use (?:)to avoid capturing the group. The complete regex then becomes ^(?:\d{1}){2}