# Help with a regular expressions and gsub

**URL:** <https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943>\
**Category:** Rails\
**Created:** [April 18, 2009, 9:35am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943 "2009-04-18T09:35:06Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 18, 2009, 9:35am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/1 "2009-04-18T09:35:06Z")

</div>

I need to use gsub in a large text. For example, I will like to search  
for ‘car’ in this text.

“Lorem ipum car. Locar ipsum,

car lorem ipusum

.Lorem car ipsum”

I will like to put ‘bike’ instead of ‘car’. Exactly I need this words,  
and not ’ bike ', for example. Also, I do not want to do ‘Lobike’ in  
‘Locar’ (it should remain ‘Locar’).

I’ll appreciate any help.

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 18, 2009, 9:39am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/2 "2009-04-18T09:39:46Z")

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On Sat, Apr 18, 2009 at 9:35 AM, John S.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I’ll appreciate any help.

Use word break matches in your regular expression as follows:

.gsub(/\bcar\b/, ‘bike’)

Andrew T.  
[http://ramblingsonrails.com](http://ramblingsonrails.com)  
[http://www.linkedin.com/in/andrewtimberlake](http://www.linkedin.com/in/andrewtimberlake)

“I have never let my schooling interfere with my education” - Mark Twain

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 19, 2009, 10:56pm UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/3 "2009-04-19T22:56:02Z")

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Andrew T. wrote:

> On Sat, Apr 18, 2009 at 9:35 AM, John S.  
> [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > I’ll appreciate any help.
> 
> Use word break matches in your regular expression as follows:
> 
> .gsub(/\bcar\b/, ‘bike’)
> 
> Andrew T.  
> [http://ramblingsonrails.com](http://ramblingsonrails.com)  
> [http://www.linkedin.com/in/andrewtimberlake](http://www.linkedin.com/in/andrewtimberlake)
> 
> “I have never let my schooling interfere with my education” - Mark Twain

Thanks. What if I have…?

“Lorem ipum car. Locar ipsum,

car lorem ipusum

.Lorem car ipsum.  
Lorem car ipsum.”

And now I want to obtain the same but I do not want to obtain bike,

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 20, 2009, 9:15am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/4 "2009-04-20T09:15:34Z")

</div>

Thanks. When I use \<\>. what I need is to obtain

bike

 and car.

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 20, 2009, 9:36am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/5 "2009-04-20T09:36:22Z")

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On Mon, Apr 20, 2009 at 9:15 AM, John S.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Thanks. When I use \<\>. what I need is to obtain
> 
> bike
> 
> and car.

I’m starting to think that regular expressions are not the best way to  
solve this.  
You should probably use an HTML parser and then do regular expression  
substitutions based on where in the DOM you are

Andrew T.  
[http://ramblingsonrails.com](http://ramblingsonrails.com)  
[http://www.linkedin.com/in/andrewtimberlake](http://www.linkedin.com/in/andrewtimberlake)

“I have never let my schooling interfere with my education” - Mark Twain

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 20, 2009, 8:09am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/6 "2009-04-20T08:09:43Z")

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On Sun, Apr 19, 2009 at 10:56 PM, John S.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > 
> 
> And now I want to obtain the same but I do not want to obtain bike,

This will start to get tricky as it becomes important to know what the  
rules really are.  
Are you wanting to avoid car surrounded by an anchor tag, or car  
surrounded by any tag.

car lorem ipusum

 will pose a problem as it's half surrounded by a tag.

so to satisfy your test case, the following works by making sure that  
car is not followed by a ‘\<’ which will work on both cases mentioned  
above.

.gsub(/\bcar\b(?=[^\<])/, ‘bike’)  
This checks that car is surrounded by a word-break but is not followed  
by a ‘\<’

If, in Ruby 1.9, I do the following:  
.gsub(/(?\<!\>)\bcar\b(?=[^\<])/, ‘bike’)  
I am now checking that car is surrounded by a word-break and is not  
preceded by a ‘\>’ and not followed by a ‘\<’ however this pattern will  
not replace “

car lorem” with “

bike lorem”

Andrew T.  
[http://ramblingsonrails.com](http://ramblingsonrails.com)  
[http://www.linkedin.com/in/andrewtimberlake](http://www.linkedin.com/in/andrewtimberlake)

“I have never let my schooling interfere with my education” - Mark Twain

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 20, 2009, 10:59am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/7 "2009-04-20T10:59:01Z")

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On Mon, Apr 20, 2009 at 10:38 AM, John S.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Thanks again for yyour help.  
> Anyway, can you explain the meaning of (?=[^\<]). I know that [^\<] means  
> no math ‘\<’, but why should I use the ‘()’ and ‘?=’ ?

(?=) is a look-ahead match that doesn’t consume the characters  
matched  
In string “cartoon”  
/car(?=[^\<])/ will only match “car” and /car[^\<]/ will match “cart”  
/car(?=[^\<])/ can actually be rewritten as /car(?!\<)/ where ?! is a  
negative look ahead

(?\<) is a look-behind match that doesn’t consume the  
characters matched (only Ruby 1.9)

Andrew T.  
[http://ramblingsonrails.com](http://ramblingsonrails.com)  
[http://www.linkedin.com/in/andrewtimberlake](http://www.linkedin.com/in/andrewtimberlake)

“I have never let my schooling interfere with my education” - Mark Twain

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<div class="post-metadata">

**Author:** ![captaffy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f04885/32.png) [@captaffy](https://www.ruby-forum.com/u/captaffy)\
**Post date:** [April 20, 2009, 10:38am UTC](https://www.ruby-forum.com/t/help-with-a-regular-expressions-and-gsub/196943/8 "2009-04-20T10:38:11Z")

</div>

Thanks again for yyour help.  
Anyway, can you explain the meaning of (?=[^\<]). I know that [^\<] means  
no math ‘\<’, but why should I use the ‘()’ and ‘?=’ ?