Regular Expressions: Constrained Repetition

• Sometimes we need text to be of a certain length. We could repeat . to enforce length.

• >

  /^.....$/.test('1234');

  Result:

  false

• >

  /^.....$/.test('12345');

  Result:

  true

• >

  /^.....$/.test('123456');

  Result:

  false

• This is awkward, especially if we want to match exactly, say, 20 characters. Fortunately, there's a better way: {curly braces}.

• >

  /^.{5}$/.test('1234');

  Result:

  false

• >

  /^.{5}$/.test('12345');

  Result:

  true

• >

  /^.{5}$/.test('123456');

  Result:

  false

• We can repeat anything in this way, not just .

• >

  /^(a|b){3}$/.test('aaa');

  Result:

  true

• >

  /^(a|b){3}$/.test('bba');

  Result:

  true

• >

  /^(a|b){3}$/.test('ab');

  Result:

  false

• By adding a comma, we can specify a range of allowed lengths.

• >

  /^.{2,3}$/.test('1');

  Result:

  false

• >

  /^.{2,3}$/.test('12');

  Result:

  true

• >

  /^.{2,3}$/.test('123');

  Result:

  true

• >

  /^.{2,3}$/.test('1234');

  Result:

  false

• We can also specify "n or more characters" by omitting the second number. For example, .{8,} means "at least eight characters".

• >

  /^[fho]{3,}$/.test('of');

  Result:

  false

• >

  /^[fho]{3,}$/.test('off');

  Result:

  true

• >

  /^[fho]{3,}$/.test('hoof');

  Result:

  true

• In some regex systems, .{,5} means "at most five characters". Unfortunately, that's not true in JavaScript's regexes. JavaScript won't tell us about our mistake either. Instead, the {,5} gets interpreted as a literal string!

• >

  /^.{,5}$/.test('12345');

  Result:

  false

• >

  // Watch out: JavaScript regexes are weird in this case!
  /^.{,5}$/.test('.{,5}');

  Result:

  true

• This is bizarre, but not a big problem. If we need five or fewer characters, we can say .{0,5}.

• >

  /^.{0,5}$/.test('1234');

  Result:

  true

• >

  /^.{0,5}$/.test('12345');

  Result:

  true

• >

  /^.{0,5}$/.test('123456');

  Result:

  false