# Regular expression **URL:** \ **Category:** Ruby\ **Created:** [August 6, 2008, 4:28pm UTC](https://www.ruby-forum.com/t/regular-expression/145165 "2008-08-06T16:28:35Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![vanight](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/a698b9/32.png) [@vanight](https://www.ruby-forum.com/u/vanight)\ **Post date:** [August 6, 2008, 4:28pm UTC](https://www.ruby-forum.com/t/regular-expression/145165/1 "2008-08-06T16:28:35Z")
Hi, I need a regular expression for a filename : it must be composed with the following letters : a to z A to Z 0 to 9 dash sign - underscore sign \_ then how can I compare a string with it : mystring =~ /myregex/ #return true or false does it works like that ? ---
**Author:** ![vanight](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/a698b9/32.png) [@vanight](https://www.ruby-forum.com/u/vanight)\ **Post date:** [August 6, 2008, 4:55pm UTC](https://www.ruby-forum.com/t/regular-expression/145165/2 "2008-08-06T16:55:02Z")
On Aug 6, 2008, at 10:26 AM, David N. wrote: > then how can I compare a string with it : > > mystring =~ /myregex/ #return true or false > > does it works like that ? Yes. Although String#=~ returns the position the match starts, or nil if there is no match. If the right-hand-side is not a Regexp, invokes obj.=~, passing str as an argument. The default =~ in Object returns false. Since a Fixnum is not nil or false, it will be “true” and both nil and false will be “false”. The only other issue is that the dash (hyphen-minus; ‘-’) is special inside a character class of regular expressions. For example, if you wanted to match a dash or digit, you could use /[-0-9]/ so the - appearing first is not treated as a character range operation. -Rob Rob B. [http://agileconsultingllc.com](http://agileconsultingllc.com) removed\_email\_address@domain.invalid