Need help with Regular Expression to Match Blood Group

Question

I\'m trying to come up with a regex that helps me validate a Blood Group field - which should accept only A[+-], B[+-], AB[+-] and O[+-].

Here\'s th

Answer 1

When you are building an alternation (e.g. (A|B|AB|O)), you should be careful with the ordering of the elements. Many regex engines will stop at the first alternate that matches (rather than the longest). If it weren't for the [-+] forcing a backtrack, (A|B|AB|O)[-+] would not work for "AB+". It is probably better to say (AB|A|B|O)[-+] (but you should check the docs for your regex engine).

Also, if you do not intend to capture the antigen for latter use, you should you use the non-capturing grouping parentheses: (?:AB|A|B|O)[-+].

Furthermore, if you want to ensure that the only thing in the string is a blood type then you need anchors to prevent it from matching only part of the string: ^(?:AB|A|B|O)[-+]$. A quick note on anchors, Depending on your regex engine, ^ may match the beginning of a line rather than the beginning of the string if you pass it a multiline-match option. Similarly, $ may match the end of a line rather than the end of a string. For this reason there are three other anchors in common (but not %100) usage: \A, \Z, and \z. If your regex engine supports them, \A always matches the start of the string, \Z matches the end of the string or a newline just before the end of the string, and \z matches only the send of the string.