Ruby: how to tell if character is upper/lowercase

Question

I'm ashamed to ask this, because it seems like it ought to be obvious, but how does one tell whether a given character in a string is upper or lowercase in Ruby? I see no obvious canned solution in the String class.

I've resorted to the following, which does not consider non-ASCII codes:

def is_lower?(c)
  c >= 'a' && c <= 'z'
end

def is_upper?(c)
  ! is_lower(c)
end

Something else I've considered is:

def is_lower?(c)
    c != c.upcase
end

Is there something more idiomatic for this?

Check [this answer](http://stackoverflow.com/a/8529619/130641) which describes a way to do this that should be Unicode-aware. — John Ledbetter, Oct 03 '12 at 17:04
@tadman- I did indicate that it does not consider non-ASCII codes. — George Armhold, Oct 03 '12 at 18:48

score 60 · Accepted Answer · answered Oct 03 '12 at 17:06

60

Use a regex pattern: [A-Z] or:

/[[:upper:]]/.match(c)

answered Oct 03 '12 at 17:06

sfell77

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This could also be added as a method in `String` with: `class String def upper? !!match(/^[[:upper:]]+$/) end end` (apologies for lack of formatting... comments are limited that way, AFAICT) – lindes May 09 '22 at 18:05

score 35 · Answer 2 · edited May 23 '17 at 12:02

35

I don't think there is something more idiomatic. The only thing you could do -- instead of passing in the string as an argument -- is monkey patch the String class:

class String
  def is_upper?
    self == self.upcase
  end

  def is_lower?
    self == self.downcase
  end
end

"a".is_upper? #=> false
"A".is_upper? #=> true

Using the method in the answer suggested by the commenter above and monkey patching String, you could do this:

class String
  def is_upper?
    !!self.match(/\p{Upper}/)
  end

  def is_lower?
    !!self.match(/\p{Lower}/)
    # or: !self.is_upper?
  end
end

edited May 23 '17 at 12:02

Community

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answered Oct 03 '12 at 17:06

Mischa

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3

I'm still shocked that Ruby, which seems to provide a method/hook for everything, does not offer this common method out of the box! – George Armhold Oct 03 '12 at 22:16
3

@CaffeineComa: Ruby tries very hard to do the right thing. In this particular case, though, it is not obvious what the right thing is, therefore Ruby doesn't do it. For example: what should your two methods return for a character which is neither upper case nor lower case? Or for a character which in some languages is considered upper case and in some languages is considered lower case? – Jörg W Mittag Oct 04 '12 at 01:44
Note also `self` is not needed in the second solution, you could simply do `!!match(/\p{Upper})`, but maybe that's not as clear to some. If you are working with ActiveSupport it's also cleaner in my opinion to use `match(/\p{Upper}/).present?` – MusikAnimal Dec 30 '15 at 04:27
1

I'd recommend using `self != self.downcase` and `self != self.upcase` for `is_upper?` and `is_lower?`, respectively, because `'?'.upcase == '?'` – Piccolo Aug 10 '18 at 21:55
@Piccolo: I agree with your identification of the shortcomings of just testing `self == self.upcase`, however I'd argue that the character-class checks (`!!match(/^[[:upper:]]+$/)` or, closer to what's here, `!!match(/^\p{Upper}+$/)`) are better, because they'll deal with a whole lot more such potential pitfalls. – lindes May 09 '22 at 18:12

score 7 · Answer 3 · answered Jul 18 '15 at 23:04

7

What does it mean for a string to be lower case? Does it mean that the string only contains lower case characters or that it doesn't contain any upper case characters? In my case I want:

"a2".is_lower? #=> true

..which leads me to:

class String

  def is_upper?
    not self.match /[[:lower:]]/
  end

  def is_lower?
    not self.match /[[:upper:]]/
  end

end

Note that /\p{Lower}/ might be better but is unsupported on Ruby 1.8.

answered Jul 18 '15 at 23:04

Neil Stockbridge

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That "not lower"/"not upper" logic makes much sense for your context; thanks for this. (I'd initially looked at this and overlooked the presence of the `not`, and was finding this flawed, but then I looked closer, and... yeah, this'll do!) – lindes May 09 '22 at 18:16

score 7 · Answer 4 · edited Aug 26 '19 at 10:34

7

Just use an if statement, e.g. if c == c.upcase or if c == c.downcase.

edited Aug 26 '19 at 10:34

Laurenz Albe

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answered Aug 26 '19 at 09:52

Andy Stuart

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`"-".upcase == "-"` is true – Fangxing Feb 10 '22 at 09:04

score 5 · Answer 5 · edited Jun 18 '17 at 02:48

5

Matching a conversion doesn't emulate the functionality of the libc isupper() and islower() functions in that both should return false for non alpha.

Ranges seem the easiest way to do this for single characters.

class String
  def islower?
    return false if self.size > 1
    ('a'..'z').include? self
  end

  def isupper?
    return false if self.size > 1
    ('A'..'Z').include? self
  end
end

edited Jun 18 '17 at 02:48

Sebastián Palma

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answered Jun 18 '14 at 10:02

Andrew Smith

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This works only for ASCII characters, whereas this question already has several answers that work well for non-ASCII characters. – Frank Schmitt Jun 18 '14 at 10:28
1

Of course, I had a specific requirement to strictly emulate the isupper() and islower() C function so I though this might match better scenarios for others where this strict compliance is required. – Andrew Smith Jun 18 '14 at 12:43
Fair enough. __dummy_text__ – Frank Schmitt Jun 18 '14 at 12:44

score 4 · Answer 6 · edited Jun 18 '17 at 02:47

4

The easiest way to verify if a character is uppercase or lowercase:

#Can be any character
char = 'a'
if char === char.capitalize then
  return 'Character is uppercase.'
else
  return 'Character is lowercase.'
end

This very simplistic if loop can determine the 'case' of a letter, by checking if it is equal to it's uppercase form. If it's already uppercase, it'll obviously be true.

edited Jun 18 '17 at 02:47

Sebastián Palma

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answered Nov 29 '13 at 20:27

crownusa

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if loop ? Why not use a begin switch ? – Martin T. May 22 '14 at 09:48
Leaving aside whether this is the easiest way or not, I think it's worth pointing out that isn't necessarily getting the "right answer" -- because it'll treat all non-alphabetic characters as uppercase, when really, they're considered to be something else. – lindes May 09 '22 at 18:21

score 2 · Answer 7 · answered May 03 '22 at 13:24

Most of the other solutions here focus on using strings. I use ruby character ordinal values to process only a single character and avoid strings.

def is_lowercase_letter? (x)
  code = x.ord
  97 <= code && code <= 122
end

Uppercase is 65 to 90.

Using 'A'.ord would not rely on you getting the numbers correct and help novice programmers understand what is happening but does not look as clean.

Ruby: how to tell if character is upper/lowercase

7 Answers7