Yet Another C# Blog

Sometimes we make life more difficult than it needs to be. Lately, I've learned a little more about Regular Expressions and "Negative Look-Around". I've used it a lot, but it only recently dawned upon me that I was not making full use of it. Case in point, take a look at my earlier regular expression article where I explain how to validate a password with multiple requirements ( Regular Expression Alternations ). This article discusses the absense of the boolean AND in regular expressions and provides a complex IF-THEN-ELSE approach to test a string for conforming to multiple "password" constraints. And my subsequent article ( Regular Expression Double Negatives ) gets more complex with an "inside-out" approach to compensate for the lack of an AND operator. But, let's go back even further to the article where I "explain" Negative Look-ahead . In this article, I provide the simple Negative Look-ahead pattern... “(?!pattern)” ....

In my last article titled "Regular Expression Alternation" we discussed Alternation as one means of performing the AND operation in a regular expression pattern. (?(expression)yes|no) But, what if you don't have this pattern in your version of RegEx? Some of you may be reading to gain knowledge that you can apply in other programming languages besides C# or the .NET environment. The pattern I described in the last article does not work in all versions of .NET Regular Expressions. In particular, it does not work in JScript Regular Expression Syntax. And, when you use tools like ASP.NET’s RegularExpressionValidator, you are implicitly using JScript. So then, what do you do? To restate the original requirement, we want to validate a string that has at least one digit, at least one uppercase letter, and at least 6 characters. The problem statement is clearly an AND problem, so it begs to be written with the Alternation pattern. But in situations where we can...

I promised in my last article that I'd have more to say about regular expression. So here you have it. Today, we look at another tough problem for Regular Expressions. Let’s consider writing a pattern to validate that a string contains at least one digit, at least one uppercase letter, and at least 6 characters. One would think that this would be easy. But here we’re faced with an AND situation and the regular expression syntax doesn't provide an AND operator. Take, for example, any validation problem that has the form… A and B and C Without an AND operation in Regex, you are almost forced to go outside of the pattern and implement the test in multiple patterns and multiple passes of your validator. That may be the best approach, or it may be impossible if you are working with a blackbox validator and must provide a single Regex pattern. I've found two approaches that can be applied to solve such problems. We'll discuss one of those approaches today. The app...

In my last article on Regular Expressions, we looked at a couple of simple expressions and 3 algorithms to use the expressions for “validating” strings. The main point of the article is that Regular Expression behavior will be confusing if not considered in the context of the algorithm being used. Please take a look at Regular Expressions. What’s that got to do with C#? to get the background for this article. In this article, we will discuss one of the more difficult Regular Expression problems. How do you use “Negative Look-ahead” ? To use Regular Expression “Negative Look-Ahead” or “Negative Look-Behind”, you have to change the way you think about pattern matching. First, the negative look-ahead takes the syntax… “(?!pattern)” In the words of the MSDN documentation… (Zero-width negative lookahead assertion.) Continues match only if the subexpression does not match at this position on the right. For example, \b(?!un)\w+\b matches words that do not begin with un. This expres...

...Only that I often need to know Regular Expressions for my C# work. However, the online help and resources seem to come up a little short. So, today I diverge a little and discuss this cryptic yet valuable ancillary topic to try to help you through your next Regex dilema. I'm not going to waste time and internet bandwidth explaining what a Regular Expression is , there are plenty of sites for those. But, I will give special thanks here to OmegaMan who compiled and posted the following on the MSDN Regex Forum... OmegaMan's .Net Regex Resources Reference I refer to it often. So let's dive right in. Whenever you are considering using regular expressions, you need to determine what kind of pattern matching problem are you trying to solve. 1) Do I want a regular expression to check a string for validity? 2) Do I want a regular expression to find certain things in my string? 3) Do I want a regular expression so that I can replace patterns in my string? There can be some...