How to check for nested square brackets?

Question

I have the input string below:

[text1][text2][text3]...[textN]

and I want to apply the following validation rule using regular expression:

The ] and [ cannot be included in other [].

For example, the next input strings are not correct:

[test1][test2[][test3]
[test1][test2]][test3] 
[test1][test2[lol][test3]
[test1][test2]lol][test3]

I need to validate the input string because I am going to split it on [] groups (again using regular expression).

I've done it before by counting, loop through the string, add 1 when you see [ and subtract 1 when you see ]. If the value is ever 2, its nested. It has other uses too, if you go through the whole string and the result isn't zero, then you have mismatched brackets. — Ron Beyer, Apr 30 '15 at 14:32
@Derek I am doing this in the context of `T-SQL` using SQL CLR implementations of .net regular expression functions. It seems to be the fastest way to validate the input string there? — gotqn, Apr 30 '15 at 14:32
Is any text allowed _outside_ square brackets? Do you want to check on that, too? — C.Evenhuis, Apr 30 '15 at 14:33
And you might want to move that logic into c# or wherever the code is BEFORE you try to write it to the database. — Derek, Apr 30 '15 at 14:34
@C.Evenhuis Yes, but I can do it after the first check, with separate regular expression. — gotqn, Apr 30 '15 at 14:35

Irwene · Accepted Answer · 2015-05-01T16:31:59.260

4

If you really want a regexp here is a quick one :

^(\[[^\[\]]+\])*$

Works on your examples

The principle here is for each bracket pair (\[.*\])* to contain any text that does NOT contains a bracket [^\[\]]+

In case you need to be able to have [test1][test2][][test3] working change the + with an * to allow the empty string to match

edited May 01 '15 at 16:31

answered Apr 30 '15 at 14:35

Irwene

score 2 · Answer 2 · edited Apr 30 '15 at 16:37

2

This should do the trick:

^(\[\w*\])*$

It means

edited Apr 30 '15 at 16:37

Irwene

answered Apr 30 '15 at 14:34

Thomas Ayoub

3

@Sidewinder94 Numbers will work, but punctuation other than underscore wouldn't. `\w` is `[A-Za-z0-9_]` – juharr Apr 30 '15 at 14:37

2 Answers2