You want to match a hexadecimal number within a certain range of numbers. You want the regular expression to specify the range accurately, rather than just limiting the number of digits.

1 to C (1 to 12: hour or month):

^[1-9a-c]$

1 to 18 (1 to 24: hour):

^(1[0-8]|[1-9a-f])$

1 to 1F (1 to 31: day of the month):

^(1[0-9a-f]|[1-9a-f])$

1 to 35 (1 to 53: week of the year):

^(3[0-5]|[12][0-9a-f]|[1-9a-f])$

0 to 3B (0 to 59: minute or second):

^(3[0-9a-b]|[12]?[0-9a-f])$

0 to 64 (0 to 100: percentage):

^(6[0-4]|[1-5]?[0-9a-f])$

1 to 64 (1 to 100):

^(6[0-4]|[1-5][0-9a-f]|[1-9a-f])$

20 to 7E (32 to 126: printable ASCII codes):

^(7[0-9a-e]|[2-6][0-9a-f])$

0 to 7F (0 to 127: 7-bit number):

^[1-7]?[0-9a-f]$

0 to FF (0 to 255: 8-bit number):

^[1-9a-f]?[0-9a-f]$

1 to 16E (1 to 366: day of the year):

^(16[0-9a-e]|1[0-5][0-9a-f]|[1-9a-f][0-9a-f]?)$

76C to 833 (1900 to 2099: year):

^(83[0-3]|8[0-2][0-9a-f]|7[7-9a-f][0-9a-f]|76[c-f])$

0 to 7FFF: (0 to 32767: 15-bit number):

^([1-7][0-9a-f]{3}|[1-9a-f][0-9a-f]{1,2}|[0-9a-f])$

0 to FFFF: (0 to 65535: 16-bit number):

^([1-9a-f][0-9a-f]{1,3}|[0-9a-f])$

There’s no difference between matching decimal numeric ranges and hexadecimal numeric ranges with a regular expression. As the previous recipe explains, split the range into multiple ranges, until each range has a fixed number of positions with independent hexadecimal digits. Then it’s just a matter of using a character class for each position, and combining the ranges with alternation.

Since letters and digits occupy separate areas in the ASCII and Unicode character tables, you cannot use the character class ‹[0-F]› to match any of the 16 hexadecimal digits. Though this character class will actually do that, it will also match the punctuation symbols that sit between the digits and the letters in the ASCII table. Instead, place two character ranges in the character class: [0-9A-F].

Another issue that comes into play is case-sensitivity. By default, regular expressions are case-sensitive. ‹[0-9A-F]› matches only uppercase characters, and ‹[0-9a-f]› matches only lowercase characters. ‹[0-9A-Fa-f]› matches both.

Explicitly typing both the uppercase and lowercase ranges in each character class quickly gets tedious. Turning on the case insensitivity option is much easier. See Recipe 3.4 to learn how to do that in your favorite programming language.

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 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.12 explains repetition.