RegEx in Ruby: Just one match?

Question

I'm trying to figure out how to check if a string matches a regular expression, but I want to know if the entire string matches just once. Here's my code but it seems absurdly long

def single_match(test_me, regex)
  ret_val = false
  test = regex.match(test_me)
  if (test.length==1 && test[0].length == test_me.length)
      ret_val = true
  end
  return ret_val
end

is there an easier way to do this?

P.S. Here's the method I'm really trying to write, since people always seem to ask why I want the gun these days:

def is_int(test_me)
  return single_match(test_me, /[0-9]*/)
end

Edit Thanks everybody. Here's where I'm really using it, but this regex stuff is always interesting to go through. Thanks for the great and educational answers.

Samuel · Accepted Answer · 2009-01-27T16:22:28.360

7

You don't need to do this, your method can be replaced by using the regular expression of /^[0-9]*$/. The ^ tells it match start of a line and $ tells it match end of the line. So it will match: start of line, 0 to any in range of 0 to 9, and finally end of line.

def is_int(test_me)
  test_me =~ /^[0-9]*$/
end

And you don't need the return statements, Ruby implicitly returns the last statement.

Edit:

It probably would be easier and look better to use the to_i instance method of String class.

def is_int(test_me)
  test_me.to_i.to_s == test_me
end

Edit: (did some tests)

Comparing the performance between the two methods shows that .to_i.to_s == way is 5% faster. So it is up to personal preference to which ever looks better and if you want to handle leading zeroes.

edited Jan 27 '09 at 16:22

answered Jan 27 '09 at 15:57

Samuel

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Regex! I like your answer and Zach Langley's answer too... THANKS. – Dan Rosenstark Jan 27 '09 at 15:59
That will match the empty string too. Consider /^\d+$/ – PEZ Jan 27 '09 at 16:02
That could be a good or bad thing. Since he used * in his question, I used it to. He may want an empty string to be seen as a number (0). – Samuel Jan 27 '09 at 16:03
@Samuel: Yes, that's why I said "consider". – PEZ Jan 27 '09 at 16:04
Was that a copy and paste, or parallel evolution? – DanSingerman Jan 27 '09 at 16:09
The former doesn't handle negatives and will match an empty string (but consider PEZ's recommendations). I'd say HermanD's solution is much more elegant. – Zach Langley Jan 27 '09 at 16:27
@Pez, yeah, didn't think of empty String. – Dan Rosenstark Jan 27 '09 at 17:24
Samuel, just 9 months later or so, this answer still works :) – Dan Rosenstark Oct 26 '09 at 17:06

score 3 · Answer 2 · answered Jan 27 '09 at 16:00

3

To do what you really want should be even simpler

def is_int(test_me)
  test_me.to_i.to_s == test_me
end

answered Jan 27 '09 at 16:00

DanSingerman

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This won't work with leading 0's, but will work with negatives. +1 – Zach Langley Jan 27 '09 at 16:06
I guess you could argue that integers shouldn't have leading zeroes (assuming that's what the function is trying to determine) – DanSingerman Jan 27 '09 at 16:07
Thanks Zach, for my purposes without leading zeroes is okay, but still... the to_i can't figure it out? Weird... – Dan Rosenstark Jan 27 '09 at 17:22
"0000123".to_i gives 123 - which makes sense to me. Hence the comparison fails when you turn that back into a string – DanSingerman Jan 27 '09 at 17:53
I get it, HermanD... interesting. But as everyone has pointed out to me, I shouldn't be doing type conversions like this anyway, so I guess it's fine... – Dan Rosenstark Jan 28 '09 at 06:00

score 2 · Answer 3 · answered Jan 27 '09 at 15:56

2

This?

def single_match(str, regex)
  str.match(regex).to_s == str
end

answered Jan 27 '09 at 15:56

Zach Langley

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score 1 · Answer 4 · answered Jan 27 '09 at 20:30

1

To answer your original question, for the sake of people finding this page in a search, "scan" will return an array of matches, so if you want to find out how many times some regexp matches, e.g. how many runs of digits there are, you can do:

mystring.scan(/\d+/).size

answered Jan 27 '09 at 20:30

glenn mcdonald

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You would then know how many RUNS of digits there are, but not whether they occupy the whole string (which was the question) or some part of it. Hence setting mystring to "3332b" is the same as "3" with your scanning regex. – Dan Rosenstark Jan 28 '09 at 05:57
Ah, ambiguity in the phrase "the entire string matches just once": "there is only one match anywhere in the string" vs "the string is a complete match"... – glenn mcdonald Jan 28 '09 at 21:24
Thanks Glenn, I was never informed of your comment, excellent point. – Dan Rosenstark Oct 26 '09 at 18:03

RegEx in Ruby: Just one match?

4 Answers4