Question

I need a regex to validate:

• just numbers
• it have to start with the number 20
• the five number needs to be 0 or 1
• the seven number needs to be 1, 2 or 3

Answer 1

@"^20\d{2}[0-1]\d[1-3]$"

Starts with 20, then 2 of any digits, then 0 or 1, then any digits, then 1, 2 or 3. If you need additional digits after the 7th, you can insert a \d* before the $:

@"^20\d{2}[0-1]\d[1-3]\d*$"

Edit: As CodeMonkey points out, \d will be interpreted in C# as an escape sequence, so be sure to use a verbatim string (as now shown above.)