Why doesn t 01-12 range work as expected

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

User · Answer

This also works      1-9   0-1  0-2      1-9  matches single digits between 1 and 9   0-1  0-2  matches double digits between 10 and 12  There are some good examples here

User · Answer

Use this   0  1-9  1 012     07  valid 7  valid 0  not match 00   not match 13   not match 21   not match   To test a pattern as 07 2018 use this      0  1-9  1 012      2-9  0-9  3        Date range between 01 2000 to 12 9999

User · Answer

A character class in regular expressions, denoted by the [...] syntax, specifies the rules to match a single character in the input. As such, everything you write between the brackets specify how to match a single character.

Your pattern, [01-12] is thus broken down as follows:

0 - match the single digit 0
or, 1-1, match a single digit in the range of 1 through 1
or, 2, match a single digit 2

So basically all you're matching is 0, 1 or 2.

In order to do the matching you want, matching two digits, ranging from 01-12 as numbers, you need to think about how they will look as text.

You have:

01-09 (ie. first digit is 0, second digit is 1-9)
10-12 (ie. first digit is 1, second digit is 0-2)

You will then have to write a regular expression for that, which can look like this:

  +-- a 0 followed by 1-9
  |
  |      +-- a 1 followed by 0-2
  |      |
<-+--> <-+-->
0[1-9]|1[0-2]
      ^
      |
      +-- vertical bar, this roughly means "OR" in this context

Note that trying to combine them in order to get a shorter expression will fail, by giving false positive matches for invalid input.

For instance, the pattern [0-1][0-9] would basically match the numbers 00-19, which is a bit more than what you want.

I tried finding a definite source for more information about character classes, but for now all I can give you is this Google Query for Regex Character Classes. Hopefully you'll be able to find some more information there to help you.

User · Answer

As polygenelubricants says yours would look for 0 1-1 2 rather than what you wish for  due to the fact that character classes  things in     match characters rather than strings

User · Answer

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       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     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     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

User · Answer

The   s in a regex denote a character class   If no ranges are specified  it implicitly ors every character within it together   Thus   abcde  is the same as  a b c d e   except that it doesn t capture anything  it will match any one of a  b  c  d  or e   All a range indicates is a set of characters   ac-eg  says  match any one of  a  any character between c and e  or g    Thus  your match says  match any one of  0  any character between 1 and 1  i e   just 1   or 2   Your goal is evidently to specify a number range  any number between 01 and 12 written with two digits   In this specific case  you can match it with 0 1-9  1 0-2   either a 0 followed by any digit between 1 and 9  or a 1 followed by any digit between 0 and 2   In general  you can transform any number range into a valid regex in a similar manner   There may be a better option than regular expressions  however  or an existing function or module which can construct the regex for you   It depends on your language

[regex] Why doesn't [01-12] range work as expected?

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