Question

I’m trying to use the range pattern [01-12] in regex to match two digit mm, but this doesn’t work as expected.

Answer 1

You seem to have misunderstood how character classes definition works in regex.

To match any of the strings 01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 11, or 12, something like this works:

0[1-9]|1[0-2]

References

• regular-expressions.info/Character Classes
  • Numeric Ranges (have many examples on matching strings interpreted as numeric ranges)

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Explanation

A character class, by itself, attempts to match one and exactly one character from the input string. [01-12] actually defines [012], a character class that matches one character from the input against any of the 3 characters 0, 1, or 2.

The - range definition goes from 1 to 1, which includes just 1. On the other hand, something like [1-9] includes 1, 2, 3, 4, 5, 6, 7, 8, 9.

Beginners often make the mistakes of defining things like [this|that]. This doesn’t “work”. This character definition defines [this|a], i.e. it matches one character from the input against any of 6 characters in t, h, i, s, | or a. More than likely (this|that) is what is intended.

References

• regular-expressions.info/Brackets for Grouping and Alternation with the vertical bar

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How ranges are defined

So it’s obvious now that a pattern like between [24-48] hours doesn’t “work”. The character class in this case is equivalent to [248].

That is, - in a character class definition doesn’t define numeric range in the pattern. Regex engines doesn’t really “understand” numbers in the pattern, with the exception of finite repetition syntax (e.g. a{3,5} matches between 3 and 5 a).

Range definition instead uses ASCII/Unicode encoding of the characters to define ranges. The character 0 is encoded in ASCII as decimal 48; 9 is 57. Thus, the character definition [0-9] includes all character whose values are between decimal 48 and 57 in the encoding. Rather sensibly, by design these are the characters 0, 1, …, 9.

See also

• Wikipedia/ASCII

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Another example: A to Z

Let’s take a look at another common character class definition [a-zA-Z]

In ASCII:

• A = 65, Z = 90
• a = 97, z = 122

This means that:

• [a-zA-Z] and [A-Za-z] are equivalent
• In most flavors, [a-Z] is likely to be an illegal character range
  • because a (97) is “greater than” than Z (90)
• [A-z] is legal, but also includes these six characters:
  • [ (91), \ (92), ] (93), ^ (94), _ (95), ` (96)

Related questions

• is the regex [a-Z] valid and if yes then is it the same as [a-zA-Z]