Simple Regular Expression (Regex) issue (.net)

Question

I'm trying to use a .NET Regex to validate the input format of a string. The string can be of the format

single digit 0-9 followed by
single letter A-Z OR 07 OR 03 or AA followed by
two letters A-Z

So 0AAA, 107ZF, 503GH, 0AAAA are all valid. The string with which I construct my Regex is as follows:

    "([0-9]{1})" +
    "((03$)|(07$)|(AA$)|[A-Z]{1})" +
    "([A-Z]{2})"

Yet this does not validate strings in which the second term is one of 03, 07 or AA. Whilst debugging, I removed the third term from the string used to construct the regex, and found that input strings of the form 103, 507, 6AA WOULD validate.......

Any ideas why, when I then put the third term back into the Regex, the input strings such as 1AAGM do not match?

Thanks Tom

Answer 1

This is because your expression requires the strings with 03, 07 and AA to end right there ($ matches the end of input). Remove the $ from these sub-expressions, and move it to the end of the expression.

"^[0-9](03|07|AA|[A-Z])[A-Z]{2}$"

Answer 2

I believe this is because you are using the "$" in the regex, which means in this case to assert position at the end of a line (at the end of the string or before a line break character). Remove it and it should work. From Regex Buddy, here is what you were doing:

([0-9]{1})((03$)|(07$)|(AA$)|[A-Z]{1})([A-Z]{2})

Options: ^ and $ match at line breaks

Match the regular expression below and capture its match into backreference number 1 «([0-9]{1})»
   Match a single character in the range between “0” and “9” «[0-9]{1}»
      Exactly 1 times «{1}»
Match the regular expression below and capture its match into backreference number 2 «((03$)|(07$)|(AA$)|[A-Z]{1})»
   Match either the regular expression below (attempting the next alternative only if this one fails) «(03$)»
      Match the regular expression below and capture its match into backreference number 3 «(03$)»
         Match the characters “03” literally «03»
         Assert position at the end of a line (at the end of the string or before a line break character) «$»
   Or match regular expression number 2 below (attempting the next alternative only if this one fails) «(07$)»
      Match the regular expression below and capture its match into backreference number 4 «(07$)»
         Match the characters “07” literally «07»
         Assert position at the end of a line (at the end of the string or before a line break character) «$»
   Or match regular expression number 3 below (attempting the next alternative only if this one fails) «(AA$)»
      Match the regular expression below and capture its match into backreference number 5 «(AA$)»
         Match the characters “AA” literally «AA»
         Assert position at the end of a line (at the end of the string or before a line break character) «$»
   Or match regular expression number 4 below (the entire group fails if this one fails to match) «[A-Z]{1}»
      Match a single character in the range between “A” and “Z” «[A-Z]{1}»
         Exactly 1 times «{1}»
Match the regular expression below and capture its match into backreference number 6 «([A-Z]{2})»
   Match a single character in the range between “A” and “Z” «[A-Z]{2}»
      Exactly 2 times «{2}»

Followed by the revised version:

([0-9]{1})((03)|(07)|(AA)|[A-Z]{1})([A-Z]{2})

Options: ^ and $ match at line breaks

Match the regular expression below and capture its match into backreference number 1 «([0-9]{1})»
   Match a single character in the range between “0” and “9” «[0-9]{1}»
      Exactly 1 times «{1}»
Match the regular expression below and capture its match into backreference number 2 «((03)|(07)|(AA)|[A-Z]{1})»
   Match either the regular expression below (attempting the next alternative only if this one fails) «(03)»
      Match the regular expression below and capture its match into backreference number 3 «(03)»
         Match the characters “03” literally «03»
   Or match regular expression number 2 below (attempting the next alternative only if this one fails) «(07)»
      Match the regular expression below and capture its match into backreference number 4 «(07)»
         Match the characters “07” literally «07»
   Or match regular expression number 3 below (attempting the next alternative only if this one fails) «(AA)»
      Match the regular expression below and capture its match into backreference number 5 «(AA)»
         Match the characters “AA” literally «AA»
   Or match regular expression number 4 below (the entire group fails if this one fails to match) «[A-Z]{1}»
      Match a single character in the range between “A” and “Z” «[A-Z]{1}»
         Exactly 1 times «{1}»
Match the regular expression below and capture its match into backreference number 6 «([A-Z]{2})»
   Match a single character in the range between “A” and “Z” «[A-Z]{2}»
      Exactly 2 times «{2}»