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

Scientific approach:
^{click on the patterns to see the example on regexplanet.com, and click on the green Java button.}

• You've already showed \d{1}{2} matches "1", and doesn't match "12", so we know it isn't interpreted as (?:\d{1}){2}.
• Still, 1 is a boring number, and {1} might be optimized away, lets try something more interesting:
  \d{2}{3}. This still only matches two characters (not six), {3} is ignored.
• Ok. There's an easy way to see what a regex engine does. Does it capture?
  Lets try (\d{1})({2}). Oddly, this works. The second group, $2, captures the empty string.
• So why do we need the first group? How about ({1})? Still works.
• And just {1}? No problem there.
  It looks like Java is being a little weird here.

• Great! So {1} is valid. We know Java expands * and + to {0,0x7FFFFFFF} and {1,0x7FFFFFFF}, so will * or + work? No:

  Dangling meta character '+' near index 0
  +
  ^

  The validation must come before * and + are expanded.

I didn't find anything in the spec that explains that, it looks like a quantifier must come at least after a character, brackets, or parentheses.

Most of these patterns are considered invalid by other regex flavors, and for a good reason - they do not make sense.