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Determine If A String Contains Only A Whole Number In Ruby

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string =~ /^\s*\d+\s*$/
Determines if there is one and only one substring in a string and that it is a whole number. The whole numbers are 0 and the counting numbers, in other words: 0, 1, 2, 3, and so on. No punctuation of any kind is allowed within the string, but the substring can optionally have leading whitespace, leading zeros, and trailing whitespace. 


<b>0</b> True. Found match for expression at position 0.
<b>nil</b> False. No match found for the expression.

<b>Explanation</b> from left to right

Use only on strings.

is the Ruby regex match operator. 

The expression is enclosed in forward slashes to compile correctly. 

<b>^</b> (circumflex or caret)
Anchor the beginning of the string. It can be replaced with <b>\A</b> because it's Ruby. The caret doesn't stand for negation here because it's not inside square brackets.

Stands for whitespace. 

0 or more of the preceding regular expression.

Stands for the digits zero through nine. <b>[0-9]</b> could have been used instead. <b>[[:digit:]]</b> could also have been substituted for the <b>\d</b>. It's POSIX 1003.2 section (6)-compliant, ugly, and little-used, but it does take "locale" into account. (What are the locales that don't use 0-9 as digits?)

1 or more of the preceding regular expression.

<b>$</b> (dollar)
Anchor the end of the string. It can be replaced with <b>\Z</b> (or <b>z</b> to anchor before a newline at the end) because it's Ruby.

<b>Determine if a string contains only a valid whole number in Ruby</b>
string =~ /^\s*[0|^0\d+]\s*$/
This can be used for the same purpose as above except leading zeros are no longer allowed. The <b>0|^0</b> inside the square brackets accomplishes this. 

The number can be zero, the <b>0</b> in the expression, but a number of two or more digits can't start with a zero, the <b>^0\d+</b>. It can be one or the other, the <b>|</b>. (The caret, <b>^</b>, only works as negation within square brackets, <b>[</b> and <b>]</b>).