Using regex to match a specific pattern

Question

I'm trying to match any strings that doesn't start with "false1" or "false2", and ends with true, but the regex isn't matching for some reason. What am I doing wrong?

$text = "start true";
$regex = "~(?:(^false1|false2).+?) true~";
if (preg_match($regex, $text, $match)) {

echo "true";    

}

Expected Result:

true

Actual Result:

null

Throw your regex into: regex101.com and look at the explanation to the right. Then you will see what your regex really does. (^(?<!false1|false2)(.*?)true$) — Rizier123
– Rizier123, Commented Oct 21, 2015 at 2:12
@Rizier123 Is that regex in your comment a solution or...? Because it doesn't match correctly. — frosty
– frosty, Commented Oct 21, 2015 at 2:17
@Rizier123 Also, I already know about that website, and knowing what it does, doesn't help me figure out how to fix it. — frosty
– frosty, Commented Oct 21, 2015 at 2:18
@Rizier123 Well, I assume you didn't trying matching with starting of "false1" and "false2"? Because it echos there too. — frosty
– frosty, Commented Oct 21, 2015 at 2:19

Avinash Raj · Accepted Answer · 2015-10-21 02:31:51Z

1

You may use negative lookahead.

^(?!false[12]).*true$

If you really want to use boundaries then try this,

^(?!false[12]\b).*\btrue$

DEMO

Update:

^(?!.*false[12]\b).*\btrue$

(?!.*false[12]\b) negative lookahead which asserts that the string would contain any char but not the sub-string false1 or false2 and it must ends with the string true, that's why we added true$ at the last.

edited Oct 21, 2015 at 2:31

answered Oct 21, 2015 at 2:19

Avinash Raj

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7 Comments

frosty Over a year ago

But if the $text is something like "start false1 true", it also echos.

Avinash Raj Over a year ago

I'm trying to match any strings that doesn't start with "false1" or "false2" . But the above does'nt starts with false1.

frosty Over a year ago

I don't want it to match ANY string that has false1, or false2 followed by true. Sorry if I wasn't really clear with that.

Avinash Raj Over a year ago

try ^(?!.*false[12]\b).*\btrue$

frosty Over a year ago

It works. But it's a really complex regex, and there's some part that I don't understand. Can you break it down, and explain it in the answer above, please? Thanks.

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