Regex Text Field Validator for iPhone

I have the following behavior of a desktop version of a software:

• a text field is validated with a regex abc|xyz
• this means that a user is allowed to type a, ab, x, xy, abc, xyz, other symbols will not be shown in a text field

I have to port such behavior to iphone. Strings a, ab, x, xy does not match to regex abc|xyz that's why matching user input will restrict typing any char to a text filed. Is there any way to match a user input as a beginning of a regex string? Ini开发者_Go百科tial regex should not be modified (^$ could be added).

The desktop version uses QRegExpValidator class from Qt. QRegExpValidator has Intermediate state that is a, ab, x, xy in my case. It uses matchedLength() to determine Intermediate state

Also I had a look through NSRegularExpression and RegexKitLite

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Have a look at PCRE - Perl Compatible Regular Expressions. It is rumored to be used in Apple products.