正则表达式匹配包含子字符串但不包含子字符串的字符串

Question

I'm trying to write a regex that will match a string that contains a certain substring, but fails if it also contains different substring. I've found this answer, but I'm not sure how to get it to work for my needs. In the interest of being specific as possible:

• Yes, it has to do this as part of the expression. I do not have access to the code that will be processing this.
• Yes, it needs to be one expression.
• It needs to work with PHP's regex flavor. I'm pretty sure it's being evaluated using preg

To give an idea of what I'm trying to do, I have a set of URLs I'm trying to filter. URLs that have "/somedir" in them I want to match, but I don't want it to match if it also has "somestring" in the URL.

So,

• www.somesite.com/somedir/index.html
• www.somesite.com/somedir/somotherdir/index.html
• www.somesite.com/somedir/somepage.html

would all match, but,

• www.somesite.com/somedir/somestring.html
• www.somesite.com/somedir/somestring/index.html

would both fail.

Answer 1

You need a regex that will accept a certain pattern only if it is not surrounded by another pattern:

~
    (?(DEFINE)
        (?<ACCEPT> must-contain-pattern)
        (?<REFUSE> must-not-contain-pattern)
    )

    ^
        (?:(?!(?&REFUSE)).)*
            (?&ACCEPT)
        (?:(?!(?&REFUSE)).)*
    $
~ux

In the DEFINE block define the ACCEPT and REFUSE pattern according to your needs and this should work.

Edit: The pattern from above tailored for the case of yours by defining the two named sub-patterns:

~
    (?(DEFINE)
        (?<ACCEPT> \Q/somedir\E)
        (?<REFUSE> \Qsomestring\E)
    )

    ^
        (?:(?!(?&REFUSE)).)*
            (?&ACCEPT)
        (?:(?!(?&REFUSE)).)*
    $
~ux