problem writing regular expression

Question

I am trying to write a regular expression that matches only any of the following:

{string}
{string[]}
{string[6]}

where instead of 6 in the last line, there could be any positive integer. Also, wherever string appears in the above lines, there could be int.

Here is the regular expression I initially wrote : {(string|int)(([]|[[0-9]])?)}. It worked well but wouldn't allow more than one digit within the square bracket. To overcome this problem, I modified it this way : {(string|int)(([]|[[0-9]*])?)}.

The modified regex seems to be having serious problems. It matches {string[][]}. Can you please tell me what causes it to match against this? Also, when I try to enclose [0-9] within paranthesis, I get an exception saying "too many )'s". Why does this happen?

Can you please tell me how to write the regular expression that would satisfy my requirements?

Note : I am tryingthis in C#

Answer 1

You need to escape the [ as it indicates a character set in regular expressions; otherwise [[0-9]*] would be interpreted as character class of [ and 0–9, * quantifier, followed by a literal ]. So:

{(string|int)(\[[0-9]*])?}

And since the quantifier * allows zero repetitions too, you don’t need the special case for an empty [].

Answer 2

You need to escape the special characters like {} and []:

\{(string|int)(\[\d*\])?\}

You might need to use [0-9] instead of \d depending on the engine you use.