Regular Expression Validator

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C#

ASP.NET

web_development

A regular expression is working in EXPRESSO environment but not in the coding why?
the following expressions gives accurate result while in EXPRESSO environment but when i use it in my coding it only validates the middle part i.e.(2[0-9]{3}[A-H]{1})
here is the regular expression:
(2[0-9]{3}[A-H]{2}) | (2[0-9]{3}[A-H]{1}) | (2[0-9]{3})