using Regex with conditions

Question

I have the problem that I can only use one Regex.IsMatch to validate a string and I have to conditions with regular expressions to be matched for the same string, I've looking and seems that I can use regex conditionals like (?(?=regex)then|else) to match both regular expressions on the string value, but I can't reach the same result

My actual code working with two regular expressions validation is:

bool flag = false;
if (Regex.IsMatch(alias, "^[a-zñA-ZÑ_0-9-.]{6}[a-zñA-ZÑ_0-9-.]*$")) 
{
    if (Regex.IsMatch(alias, "[^0-9.-]+"))
    {
        flag = true;
    };
};

And what i want to achieve is something like this:

if (Regex.IsMatch(alias, "(?(^[a-zñA-ZÑ_0-9-.]{6}[a-zñA-ZÑ_0-9-.]*$)([^0-9.-]+))"))
{
    // Some code...
};

so if you can give me some guidance or check if my regular expression is right using the conditional in regexp it would appreciated

It might be better if you give us an example of what you're trying to match rather than just what appears to work and what doesn't. It's hard to tell what you're trying to achieve from the code you've posted. — Bennor McCarthy
– Bennor McCarthy, Commented Aug 12, 2010 at 20:09
For example: Valid: 12pl58 , -sdfgl.fdo , -1313.5m64 , Name_is_this Invalid: juan1 , -1523.32 , 1523.3002 thanks for the help — David
– David, Commented Aug 12, 2010 at 21:05

Alan Moore · Accepted Answer · 2010-08-13 02:55:36Z

2

What you need is a lookahead, not a conditional. Observe:

^(?=.*[a-zñA-ZÑ_])[a-zñA-ZÑ_0-9.-]{6,}$

The lookahead - (?=.*[a-zñA-ZÑ_]) - asserts that there is at least one of the listed characters somewhere in the string. When it's done, the match position returns to the beginning of the string so the "real" regex can match the whole thing.

For more information about lookaheads: http://www.regular-expressions.info/lookaround.html

answered Aug 13, 2010 at 2:55

Alan Moore

75.6k13 gold badges110 silver badges161 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Mikael Svenson · Accepted Answer · 2010-08-13 14:11:23Z

2

Your first regexp:

^[a-zñA-ZÑ_0-9-.]{6}[a-zñA-ZÑ_0-9-.]*$

says that 0-9.- is allowed, while the second one says 0-9.- is not allowed. Why not just change the first one to:

^[a-zñA-ZÑ_]{6}[a-zñA-ZÑ_]*$

and even simpler:

^[a-zñA-ZÑ_]{6,}$

since both parts are equal

[Edit - code sample]

Alan's answer is probably the most elegant, but you could also achieve it like this:

bool flag = false;
Match m = Regex.Match(alias, "(?<main>^[a-zñA-ZÑ_0-9-.]{6,}$)|(?<char>[^0-9.-])");
if (m.Groups["main"].Success && m.Groups["char"].Success)
{
    flag = true;
};

That said, it's often better to break up a regular expression into several parts imo. It makes it more readable, and each expression does one thing. As long as you're not executing the expression a million times a second the overhead of running several Regexp's is minuscule.

edited Aug 13, 2010 at 14:11

answered Aug 12, 2010 at 20:02

Mikael Svenson

39.8k8 gold badges76 silver badges80 bronze badges

3 Comments

David Over a year ago

thanks for helping, the fact is that the string I'm parsing can have numbers, but I must validate that it has at least one char that isn't a number, and thanks for the simplifier regexp

Alan Moore Over a year ago

When the second regex is applied, the string has already been matched by the first, so [^0-9.-]+ just ensures that at least one of the characters in the string is a letter or underscore rather than a digit, dot or dash.

Mikael Svenson Over a year ago

@alan: true. I will fix my answer once i get to a computer. Writing code on my mobile is cumbersome.

Collectives™ on Stack Overflow

using Regex with conditions

2 Answers 2

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related