# Regular expression NOT operator

**URL:** <https://www.ruby-forum.com/t/regular-expression-not-operator/175072>\
**Category:** Ruby\
**Created:** [September 7, 2009, 11:07am UTC](https://www.ruby-forum.com/t/regular-expression-not-operator/175072 "2009-09-07T11:07:22Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![basic](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/58956e/32.png) [@basic](https://www.ruby-forum.com/u/basic)\
**Post date:** [September 7, 2009, 11:07am UTC](https://www.ruby-forum.com/t/regular-expression-not-operator/175072/1 "2009-09-07T11:07:22Z")

</div>

Regex is something I’ve managed to ignore for some time, only learning  
bits when needed.

I’m trying to do code highlighting, so I’ve been using Reg to find parts  
of the code  
I’ve run into an issue with comments

# A comment can contain control statements like if and Constants

things like if and a Constant get picked up by my reg, I’m trying to put  
a not operator in my regex but cant seem to get it to work.

so for constants I search /([A-Z].\*?\b)/  
how can I add “when theres no # on the left”?

I’ve been trying to use the ?! as the not operator. (is that even a not  
operator)  
this is what i’ve been trying /([A-Z].\*?\b)(?!)/

anyhelp would be greatly appreated.

thanks  
Phil.

---

<div class="post-metadata">

**Author:** ![basic](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/58956e/32.png) [@basic](https://www.ruby-forum.com/u/basic)\
**Post date:** [September 7, 2009, 12:36pm UTC](https://www.ruby-forum.com/t/regular-expression-not-operator/175072/2 "2009-09-07T12:36:16Z")

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Hi –

On Mon, 7 Sep 2009, Phil Cooper-king wrote:

> a not operator in my regex but cant seem to get it to work.
> 
> so for constants I search /([A-Z].\*?\b)/  
> how can I add “when theres no # on the left”?
> 
> I’ve been trying to use the ?! as the not operator. (is that even a not  
> operator)  
> this is what i’ve been trying /([A-Z].\*?\b)(?!)/

There’s negative look-behind in Oniguruma, but you’re going to run  
into some difficulties anyway, I suspect. For example:

x = 3  
puts “This has no comments, and x is #{x}” if x \< 5

My advice would be to keep it (relatively) straightforward by doing  
something like this as you scan the lines of text:

comment\_re = /^\s\*#/

if comment\_re.match(line)  
# treat line as a comment  
else  
# line is not a comment  
end

David

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<div class="post-metadata">

**Author:** ![basic](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/58956e/32.png) [@basic](https://www.ruby-forum.com/u/basic)\
**Post date:** [September 8, 2009, 9:49am UTC](https://www.ruby-forum.com/t/regular-expression-not-operator/175072/3 "2009-09-08T09:49:25Z")

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> My advice would be to keep it (relatively) straightforward by doing  
> something like this as you scan the lines of text:
> 
> comment\_re = /^\s\*#/
> 
> if comment\_re.match(line)  
> # treat line as a comment  
> else  
> # line is not a comment  
> end

Thanks David.A, I’ll give Oniguruma a look but I think I’ll go with your  
suggestion

Thanks G\_F\_ for pointing out my mistake.

---

<div class="post-metadata">

**Author:** ![basic](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/58956e/32.png) [@basic](https://www.ruby-forum.com/u/basic)\
**Post date:** [September 7, 2009, 7:31pm UTC](https://www.ruby-forum.com/t/regular-expression-not-operator/175072/4 "2009-09-07T19:31:28Z")

</div>

Phil Cooper-king wrote:  
[…]

> so for constants I search /([A-Z].\*?\b)/

This regex is not going to match a constant. It matches any upper-case  
letter followed by a non-greedy wildcard followed by a word boundary.

A constant has to begin with an upper-case letter, possibly followed by  
mixed-case letters, numbers and underscores (“\_”).

The following show the problem: The first two are your regex, and the  
second two show a fix.

```
/([A-Z].*?\b)/ =~ 'noT a constant' # => 2
/([A-Z].*?\b)/ =~ 'a Constant' # => 2

/\b([A-Z]\w*\b)/ =~ 'noT a constant.' # => nil
/\b([A-Z]\w*\b)/ =~ 'a Constant.' # => 2

```

The # =\> at the end of the line show where the match occurred. The first  
set shows a non-constant having a false-positive.

Regex are extremely powerful, but you have to think out what can go  
wrong with them. When you are searching you can get false-positives  
easily. If you are searching and replacing, you can get destroyed  
content.

Also, “?!” is not a NOT operator, it’s a negative look-ahead. A match  
succeeds if the initial condition matches followed by no match.

[http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html#UN](http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html#UN)