Regex - How to select everything when the word is not contained

Question

Say that I have this text

I need to think of something to write here. I am an alumni of the college. The blue cat is blue the red cat is red.

I want to be able to select everything if the word "student" (ignoring case) does NOT exist in the text. Therefore, I need something to go beyond the first line. I originally had something like

/(?:[^student]).*/

but it isn't working correctly. I am not sure what "flavor" of Regex I am using but it is with PHP and in the backend of a Drupal site.

Thank you!

What about "If matched, use match, if not use whole string?" — tadman, Feb 07 '18 at 18:21
You clearly don't understand what `[^student]` means in a regex. `[]` is not used to group a string, it's used to specify a character class. — Barmar, Feb 07 '18 at 18:22
@ctwheels those both work but I think the problem I'm getting is from drupal now. — Mona NM, Feb 07 '18 at 18:30
@ctwheels I actually think these aren't working for me, but not sure why. They also don't work correctly when I check using regexr.com — Mona NM, Feb 07 '18 at 18:44
@MonaNM it's a negative lookahead ensuring what follows doesn't match. So having `.*student` will ensure `student` doesn't exist anywhere in the string. — ctwheels, Feb 07 '18 at 18:45

score 0 · Accepted Answer · answered Feb 07 '18 at 18:54

Problem

As suggested by ctwheels comment, the regexp you have written (?:[^student]).* is not doing what you think it does. Lets break down your attempt to illustrate the problem:

(?: ) <--- this part is a non-capturing group

[^student] <--- this part matches a single character not present in the list

.* <--- matches any character (except for line terminators)

I.e, your bracket expression is the same as [^tneduts] or [^neduts]or even [^denstu]. It doesn't look for a word!

Solution

What you want to do instead, is using a negative lookahead. like this ^(?!.*student).*$. Let's break it down.

^ <--- position at start of the string

(?!.*student) <--- look for anything followed by student, don't match if found

.* <--- take everything on the line unless the negative lookahead is in effect.

$ <--– position at the end of the string

is there a way I could replace .* in your solution to get it to search and select multiple lines? — Mona NM, Feb 07 '18 at 19:01
You could try to match newlines explicitly but I avoid regexp for multiline parsing. If you have longer texts I suggest you look for the word 'student' in the entire file instead using a simple line by line character matcher like [strpos()](http://php.net/manual/en/function.strpos.php). — jjabba, Feb 07 '18 at 19:20

Regex - How to select everything when the word is not contained

1 Answers1