Ranges

Writing a regex matching a number in a certain range can be quite difficult. For example, the following regex matches a number between 0 and 255:

(?:2(?:5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])

This has many downsides:

  • It’s not readable
  • It’s difficult to come up with
  • It’s easy to make a mistake somewhere
  • It’s inefficient; a typical regex engine needs to backtrack in several places

Pomsky solves these problems with its range syntax:

range '0'-'255'

Pomsky creates a DFA (deterministic finite automaton) from this, so the generated regex is optimal in terms of matching performance. Since the algorithm for creating this regex is extensively tested, you can also rely on it’s correctness. Here’s the regex generated from range '0'-'255':

0|1[0-9]{0,2}|2(?:[0-4][0-9]?|5[0-5]?|[6-9])?|[3-9][0-9]?

Different bases

Pomsky can generate ranges in various bases. For example, to match hexadecimal numbers in a certain range, you might write:

range '10F'-'FFFF' base 16

This generates this regex:

1(?:0(?:[0-9a-eA-E][0-9a-fA-F]|[fF][0-9a-fA-F]?)|[1-9a-fA-F][0-9a-fA-F]{1,2})|[2-9a-fA-F][0-9a-fA-F]{2,3}

Leading zeroes

If you wish to support leading zeros, this is easy to achieve by putting '0'* in front:

'0'* range '0'-'1024'

If the number should have a certain length, with leading zeroes added when necessary, pomsky has a special syntax for this:

range '0000'-'1024'

This matches numbers in the specified range with exactly 4 digits, such as 0110 or 0026.

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