IIS UrlRewrite RegEx differences with .NET RegEx

Question

I have a valid RegEx pattern in .NET:

(?>.*param1=value1.*)(?<!.*param2=\d+.*) which matches if:

query string contains param1=value1
but does not contain param2= a number

It works in .NET. However IIS URLRewrite complains that it is not a valid pattern.

Can I not use zero-width negative look behind (?<! ) expressions with IIS URLRewrite?

Note that I tried to apply this pattern both in web.config (properly changing < and > to < and > respectively, as well as in the IIS Manager - all without success.

Can't you simplify it as `^(?!.*param2=\d).*param1=value1`? Or if you need to match the whole string, add trailing `.*`: `^(?!.*param2=\d).*param1=value1.*` — Wiktor Stribiżew, Jul 19 '16 at 07:10
@WiktorStribiżew Thanks! This works! However the question remains as to the differences. I thought that the 2 RegEx engines used are equivalent. — Fit Dev, Jul 19 '16 at 07:16

score 2 · Accepted Answer · answered Jul 19 '16 at 07:16

IIS URLRewrite default regex syntax is ECMAScript, that is not compatible with .NET regex syntax. See URL Rewrite Module Configuration Reference:

ECMAScript – Perl compatible (ECMAScript standard compliant) regular expression syntax. This is a default option for any rule.

You cannot use a lookbehind at all, you will have to rely on lookaheads only:

^(?!.*param2=\d).*param1=value1.*

Pattern explanation:

^ - start of string
(?!.*param2=\d) - if there is param2= followed with a digit (\d) after 0+ characters other than a newline fails the match (return no match)
.*param1=value1.* - match a whole line that contains param1=value1

You can enhance this rule by adding \b around param1=value1 to only match it as a whole word.

IIS UrlRewrite RegEx differences with .NET RegEx

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