Reverse RegExp from user entered string ( C#)

Question

Is it possible to generate regular expressions from a user entered string? Are there any C# libraries to do this?

For example a user enters a string e.g. ABCxyz123 and the C# code automatically generates [A-Z]{3}[a-z]{3}\d{3}. This is a simple string but we could have more complicated strings like

MON-0123/AB/5678-abc 2/7

Or

1234-678/abc::1234ABC?246

I already have a string tokeniser (from a previous stackoverflow question) so I could construct a regex from the list of tokens.

But I was wondering if there is a lib or C# code out there that’ll do it.

Edit: Important, I should of also said: It's not the actual character in the string that are important but the type of character and how many.

e.g A user could enter a "pattern" string of ABCxyz123.

This would be interpreted as

3 upper case alphas followed by

3 lower case alphas followed by

3 digits

So other users (when complied) must enter strings that match that pattern [A-Z]{3}[a-z]{3}\d{3}., e.g. QAZplm789

It's the format of user entered strings that's need to be checked not the actual content if that makes sense

How do you distinguish if ABCxyc should resolve to A-Za-z or A-Cx-z? — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Nov 14, 2014 at 10:34
Something like this would be stupid to do since it wont work because [A-Z]{3}[a-z]{3}\d{3} could be a vastly difrent string then the one your entered you would need to create many rules for this and i see no point in doing this since you can just use regex itself — Vajura
– Vajura, Commented Nov 14, 2014 at 10:43
I agree with other posters, this seems like a really strange solution, which got me curious... Could you elaborate a little more on what problem you're trying to solve? — Daniel Persson
– Daniel Persson, Commented Nov 14, 2014 at 10:51
Upvoted because this is an interesting question which provides use case examples. This may be better suited for programmers.SE though, its a broad design question rather than a specific implementation. — Freiheit
– Freiheit, Commented Nov 14, 2014 at 10:52

Community · Accepted Answer · 2017-05-23 12:30:03Z

Jerry has a related link creating a regular expression for a list of strings

There are a few other links off this.

I'm not trying to do anything complicated e.g NLP etc.

I could use C# expression builder and dynamic linq at a push, but that seems overkill and a code maintainable nightmare .

I'll write my own "simple" regex builder from the tokenized string.

Example Use Case:

An admin office user where I work could setup the string patterns for each field by typing a string pattern, My code converts this to a regex, I store these in a database.

E.g: Field one requires 3 digits at the start. If there are 2 digits then send to workflow 1 if 3 then send to workflow 2. I could simply check the number of chars by substr or what ever. But this would be a concrete solution. I am trying to do this generically for multiple documents with multiple fields. Also, each field could have multiple format checkers.

I don't want to write specific C# checks for every single field in numerous documents.

I'll get on with it, should keep me amused for a couple of days.

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