An integer number is a contiguous series of one or more digits, each between zero and nine. We can easily represent this with a character class (Recipe 2.3) and a quantifier (Recipe 2.12): ‹[0-9]+›.

Beyond being a series of digits, the number must also stand alone. A4 is a paper size, not a number. There are several ways to make sure your regex only matches pure numbers.

If you want to check whether your string holds nothing but a number, simply put start-of-string and end-of-string anchors around your regex. ‹\A› and ‹\Z› are your best option, because their meaning doesn’t change. Unfortunately, JavaScript doesn’t support them. In JavaScript, use ‹^› and ‹$›, and make sure you don’t specify the /m flag that makes the caret and dollar match at line breaks. In Ruby, the caret and dollar always match at line breaks, so you can’t reliably use them to force your regex to match the whole string.

When searching for numbers within a larger body of text, word boundaries (Recipe 2.6) are an easy solution. When you place them before or after a regex token that matches a digit, the word boundary makes sure there is no word character before or after the matched digit. For example, ‹4› matches 4 in A4. ‹4\b› does too, because there’s no word character after the 4. ‹\b4› and ‹\b4\b› don’t match anything in A4, because ‹\b› fails between the two word characters A and 4. In regular expressions, word characters include letters, digits and underscores.

If you include nonword characters such as plus or minus signs or whitespace in your regex, you have to be careful with the placement of word boundaries. To match +4 while excluding +4B, use ‹\+4\b› instead of ‹\b\+4\b›. The latter does not match +4, because there’s no word character before the plus in the subject string to satisfy the word boundary. ‹\b\+4\b› does match +4 in the text 3+4, because 3 is a word character and + is not.

‹\+4\b› only needs one word boundary. The first ‹\b› in ‹\+\b4\b› is superfluous. When this regex matches, the first ‹\b› is always between a + and a 4, and thus never excludes anything. The first ‹\b› becomes important when the plus sign is optional. ‹\+?\b4\b› does not match the 4 in A4, whereas ‹\+?4\b› does.

Word boundaries are not always the right solution. Consider the subject text $123,456.78. If you iterate over this string with the regex ‹\b[0-9]+\b›, it’ll match 123, 456, and 78. The dollar sign, comma, and decimal point are not word characters, so the word boundary matches between a digit and any of these characters. Sometimes this is what you want, sometimes not.

If you only want to find integers surrounded by whitespace or the start or end of a string, you need to use lookaround instead of word boundaries. ‹(?=$|\s)› matches at the end of the string or before a character that is whitespace (whitespace includes line breaks). ‹(?<=^|\s)› matches either at the start of the string, or after a character that is whitespace. You can replace ‹\s› with a character class that matches any of the characters you want to allow before or after the number. See Recipe 2.16 to learn how lookaround works.

Perl and Python support lookbehind, but they don’t allow alternatives of different length inside lookbehind. Since ‹^› is zero-length and ‹\s› matches a single character, we have to put the ‹^› alternative outside the lookbehind. Thus ‹(?<=^|\s)› becomes ‹(?:^|(?<=\s))› for Perl and Python. These two regexes are functionally identical. The latter just takes a bit more effort on the keyboard.

JavaScript and Ruby 1.8 don’t support lookbehind. You can use a normal group instead of lookbehind to check if the number occurs at the start of the string, or if it is preceded by whitespace. The drawback is that the whitespace character will be included in the overall regex match if the number doesn’t occur at the start of the string. An easy solution to that is to put the part of the regex that matches the number inside a capturing group. The fifth regex in the section captures the whitespace character in the first capturing group and the matched integer in the second capturing group.

All the other recipes in this chapter show more ways of matching different kinds of numbers with a regular expression.

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. Recipe 2.16 explains lookaround.