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simplest way to check if a given string is a positive integer using regex

Posted on 2009-05-04
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Last Modified: 2012-05-06
This seems to be simple and yet I can't find anything other than complicated expressions I found on other sites.

Basically I need a regular expression that will check if a given string is an integer 1 or above.  So, for example, [1-9] would ensure that it's an integer 1 through 9.  But how would I check for an integer from 1 and up?
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Question by:aturetsky
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by:Terry Woods
ID: 24300353
Like this (doesn't allow leading zeroes):
[1-9][0-9]*
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by:Terry Woods
ID: 24300364
You'll probably want to make sure you don't match on a partial string too actually, so you'll need to match the start and end of the string with ^ and $ respectively:
^[1-9][0-9]*$
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by:aturetsky
ID: 24300552
Hmm, your first solution seems to work as well and doesn't appear to match on a partial string (which is as desired).   Does the need for ^ and $ depend on what language and api is being used?  I am using java and am doing

inputString.matches([1-9][0-9]*)

and it (as desired) returns false for partial matches, so for "blah45" or "45blah" it returns false, but for "45" it returns true.

Why then would we need the begin and end characters?
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by:ghostdog74
ID: 24300728
this is simple you don't need regular expression. convert the string to number (if necessary, if not depending on your language will do it for you automatically), then check if its greater than 0, eg  if int(string) > 0. i can't see why you need a regular expression for that simple task.
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by:aturetsky
ID: 24300799
ghostdog74:  I don't want to do it via exception handling (I am using java).
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by:CEHJ
ID: 24302107
You can simplify that somewhat to
boolean isPositive = s.matches("\\d+");

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by:Maciej S
ID: 24302273
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by:aturetsky
ID: 24305090
CEHJ - that looks like the best solution thus far - but it doesn't exclude 0 and, as mentioned, I need 1 and up.  Any thoughts?
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CEHJ earned 500 total points
ID: 24305839
Sorry - missed that. Of course you could do the following and save yourself one character over the first post ;-) (You don't need to worry about partial matching - it's not allowed)


inputString.matches("[1-9]\\d*");

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by:CEHJ
ID: 24764769
:-)
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