You want to find various kinds of integer decimal numbers in a larger body of text, or check whether a string variable holds an integer decimal number. The number must not have a leading zero, as only octal numbers can have leading zeros. But the number zero itself is a valid decimal number.

Find any positive integer decimal number without a leading zero in a larger body of text:

(0|[1-9][0-9]*)

Check whether a text string holds just a positive integer decimal number without a leading zero:

A(0|[1-9][0-9]*)

^(0|[1-9][0-9]*)$

Recipe 6.1 shows a lot of solutions for matching integer decimal numbers, along with a detailed explanation. But the solutions in that recipe do not take into account that in many programming languages, numbers with a leading zero are octal numbers rather than decimal numbers. They simply use ‹[0-9]+› to match any sequence of decimal digits.

The solutions in this recipe exclude numbers with a leading zero, while still matching the number zero itself. Instead of matching any sequence of decimal digits with ‹[0-9]+›, these regular expressions use ‹0|[1-9][0-9]*› to match either the digit zero, or a decimal number with at least one digit that does not begin with a zero. Since the alternation operator has the lowest precedence of all regular expression operators, we use a group to make sure the anchors and word boundaries stay outside of the alternation.

Recipe 6.4 has solutions for matching octal numbers.

Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.3 explains character classes. Recipe 2.5 explains anchors. Recipe 2.6 explains word boundaries. Recipe 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.12 explains repetition.