Regular expression : validate numbers greater than 0, with or without leading zeros

Question

I need a regular expression that will match strings like T001, T1, T012, T150 ---- T999.

I made it like this : [tT][0-9]?[0-9]?[0-9], but obviously it will also match T0, T00 and T000, which I don't want to.

How can I force the last character to be 1 if the previous one or two are zeros ?

Answer 1

Quite easy using a negative lookahead: ^[tT](?!0{1,3}$)[0-9]{1,3}$

Explanation

^               # match begin of string
[tT]            # match t or T
(?!             # negative lookahead, check if there is no ...
    0{1,3}      # match 0, 00 or 000
    $           # match end of string
)               # end of lookahead
[0-9]{1,3}      # match a digit one or three times
$               # match end of string

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