Why does this regex work in JavaScript, but not C#?

Question

Expression

var regex = new Regex(@"{([A-z]*)(([^]|:)((\\:)|[^:])*?)(([^]|:)((\\:)|[^:])*?)}");

Breakdown

The expression is [crudely] designed to find tokens within an input, using the format: {name[:pattern[:format]]}, where the pattern and format are optional.

{
  ([A-z]*) // name
  (([^]|:)((\\:)|[^:])*?) // regex pattern
  (([^]|:)((\\:)|[^:])*?) // format
}

Additionally, the expression attempts to ignore escaped colons, thus allowing for strings such as {Time:\d+\:\d+\:\d+:hh\:mm\:ss}

Question

When testing on RegExr.com, everything works sufficiently, however when attempting the same pattern in C#, the input fails to match, why?

(Any advice for general improvements to the expression are very welcome too)

Hi, because they are different a bit. According docs: In .NET, regular expression patterns are defined by a special syntax or language, which is compatible with Perl 5. So, try debug your regex here, thery are Perl and Js regex implementations; — slesh
– slesh, Commented Aug 22, 2017 at 16:53
[A-z] matches more than letters.Every character that is between Z and a in ASCII table. — Toto
– Toto, Commented Aug 22, 2017 at 16:53
Can you maybe give two examples; one that works in both and one that fails in C#? — Sach
– Sach, Commented Aug 22, 2017 at 16:53
[^] isn’t valid in .NET regular expressions. (It should throw an exception, though, I think?) — Ry-
– Ry- ♦, Commented Aug 22, 2017 at 16:55

Wiktor Stribiżew · Accepted Answer · 2017-08-22 17:36:55Z

6

The [^] pattern is only valid in JavaScript where it matches a not nothing, i.e. any character (although in ES5, it does not match the chars from outside the BMP plane). In C#, it is easy to match any char with . and passing the RegexOptions.Singleline modifier. However, in JS, the modifier is not supported, but you may match any char with [\s\S] workaround pattern.

So, the minimum change you need to make to make both compatible in both regex flavors is to change ([^]|:) to [\s\S] because there is no need to use a : as an alternative (since [\s\S] will already match a colon).

Also, do not use [A-z] as a shortcut to match ASCII letters. Either use [a-zA-Z] or [a-z] and pass a case insensitive modifier.

So, you might consider writing the expression as

{([A-Za-z]*)([\s\S]((\\:)|[^:])*?)([\s\S]((\\:)|[^:])*?)}

See a .NET regex test and a JS regex test.

Surely, there may be other enhancements here: remove redundant groups, add support for any escape sequences (not just escaped colons), etc., but it is out of the question scope.

answered Aug 22, 2017 at 17:36

Wiktor Stribiżew

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1 Comment

Richard Over a year ago

Clearly explained, and very informative; much appreciated. As a follow up to your recommendation, I've removed redundant groups.

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