# Simple Regexp help

**URL:** <https://www.ruby-forum.com/t/simple-regexp-help/157062>\
**Category:** Ruby\
**Created:** [January 7, 2009, 12:50am UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062 "2009-01-07T00:50:45Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 12:50am UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/1 "2009-01-07T00:50:45Z")

</div>

How can I test a word to make sure it ONLY contains certain characters?

Say i have an expression like /[A-Z]/i

How could i have “Testing” pass but “Testing123” fail?

PS. I can not dynamically create the expression so it looks like this  
/[A-Z{10}]/

---

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 2:01am UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/2 "2009-01-07T02:01:58Z")

</div>

Joe B. wrote:

> How can I test a word to make sure it ONLY contains certain  
> characters?
> 
> Say i have an expression like /[A-Z]/i
> 
> How could i have “Testing” pass but “Testing123” fail?
> 
> PS. I can not dynamically create the expression so it looks like this  
> /[A-Z{10}]/

If you only want a-z, set the character class to start and end the  
string, otherwise it’ll match anything with a character that’s in the  
alphabet, regardless of what follows it. I.e., /^[a-z]+$/i will only  
match a-z characters from start to end. You can then change the  
character class to whatever you wish. ^ is the start of the string and  
$ is the end of the string. + is one or more characters, so ^[a-z]+$ is  
one of more characters in the character class [], being a-z, and noting  
else.

---

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 2:17am UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/3 "2009-01-07T02:17:36Z")

</div>

Tim G. wrote:

> Joe B. wrote:
> 
> > How can I test a word to make sure it ONLY contains certain  
> > characters?
> > 
> > Say i have an expression like /[A-Z]/i
> > 
> > How could i have “Testing” pass but “Testing123” fail?
> > 
> > PS. I can not dynamically create the expression so it looks like this  
> > /[A-Z{10}]/
> 
> If you only want a-z, set the character class to start and end the  
> string, otherwise it’ll match anything with a character that’s in the  
> alphabet, regardless of what follows it. (…)

You can also test for the opposite, anything not in the range a-z.

class String  
def all\_letters?  
(self =~ /[^a-z]/i).nil?  
end  
end

puts “Testing”.all\_letters?

# =\> true

puts “Tes34ting”.all\_letters?

# =\> false

hth,

Siep

---

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 6:00pm UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/4 "2009-01-07T18:00:08Z")

</div>

Robert thanks \A and \z is what I was looking for.

Can you explain though what that does though? I can not seem to find any  
relevant information on it.

---

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 6:30pm UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/5 "2009-01-07T18:30:24Z")

</div>

On Wed, Jan 7, 2009 at 5:59 PM, Joe B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Robert thanks \A and \z is what I was looking for.
> 
> Can you explain though what that does though? I can not seem to find any  
> relevant information on it.

\A matches the beggining of the string and \z matches the end of the  
string.  
If you don’t “anchor” the regexp with those, what you match can match  
anywhere within the string, even a substring. So for example:

irb(main):001:0\> a = “abcdef”  
=\> “abcdef”  
irb(main):005:0\> a =~ /bc/  
=\> 1

This means that the regexp /bc/ has matched the string at position  
one. Notice now if you anchor it that it doesn’t match:

irb(main):006:0\> a =~ /\Abc\z/  
=\> nil  
irb(main):007:0\> a =~ /\Aabcdef\z/  
=\> 0

[www.rubular.com](http://www.rubular.com) has a regular expression editor, where you can test,  
and also a quick reference guide and a link to an online copy of the  
pickaxe, although to be honest, not much is said about \A and \z  
specifically.

You can also search here for more info about regexps:

> **[Regular-Expressions.info - Regex Tutorial, Examples and Reference - Regexp...](https://www.regular-expressions.info/)**
>
> At Regular-Expressions.info you will find a wide range of in-depth information about a powerful search pattern language called regular expressions.

Hope this helps,

Jesus.

---

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 7:15pm UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/6 "2009-01-07T19:15:52Z")

</div>

Robert K. wrote:

> irb(main):001:0\> s = “foo\n”  
> =\> “foo\n”  
> irb(main):002:0\> /^[a-z]+$/i =~ s  
> =\> 0

Not knowing exactly what the OP wanted, it was only a quick example, so  
what is “safe” is relative. Anyway, the use of \A and \z is indeed  
probably a better example regardless of not knowing exactly what they  
want.

---

<div class="post-metadata">

**Author:** ![joe-black](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/87869e/32.png) [@joe-black](https://www.ruby-forum.com/u/joe-black)\
**Post date:** [January 7, 2009, 8:45am UTC](https://www.ruby-forum.com/t/simple-regexp-help/157062/7 "2009-01-07T08:45:29Z")

</div>

On 07.01.2009 01:17, Tim G. wrote:

> > /[A-Z{10}]/
> 
> If you only want a-z, set the character class to start and end the  
> string, otherwise it’ll match anything with a character that’s in the  
> alphabet, regardless of what follows it. I.e., /^[a-z]+$/i will only  
> match a-z characters from start to end.

Note, this is not totally safe:

irb(main):001:0\> s = “foo\n”  
=\> “foo\n”  
irb(main):002:0\> /^[a-z]+$/i =~ s  
=\> 0

As you can see, even a string with a newline at the end passes. This  
version is safer because anchors do actually use start and end of the  
string:

irb(main):003:0\> /\A[a-z]+\z/i =~ s  
=\> nil

> You can then change the  
> character class to whatever you wish. ^ is the start of the string and  
> $ is the end of the string.

Actually ^ is a line start and $ is a line end (see above). For proper  
anchoring string start and end you need \A and \z.

> - is one or more characters, so ^[a-z]+$ is  
> one of more characters in the character class [], being a-z, and noting  
> else.

Note also that, depending on definition of legal string the expression  
should probably contain \* instead of + because also the empty string  
does not contain any illegal characters.

Kind regards

robert