How do you use a variable value within a regular expression pattern properly?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks!
If I have the following code:

$myvar="one";
$string="oneREC:riugfryfgfyug";
[download]

and I want to do this pattern matching:

if($string=~/^$myvarREC:(.*)/)
[download]

it does not work because REC is right after the value of $myvar. Only if I separate them, e.g. $string="one#REC:riugfryfgfyug";
it does the pattern matching. Can you tell me what I am doing wrong?

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Re: How do you use a variable value within a regular expression pattern properly? (updated) by haukex (Archbishop) on Sep 11, 2017 at 11:43 UTC
As an alternative to what 1nickt showed, there's also the `/x` modifier that allows you to insert whitespace in your regex, which also improves readability: `if( $string=~/ ^ $myvar REC: (.) /x )` [download] Also, Mind the meta!, that means if your `$myvar` contains any characters like the dot (`.`) and you don't want these to be interpreted as regex special characters, or any whitespace in the variable to be ignored under `/x`, then you need to wrap your string in `\Q...\E` (see also quotemeta): `if( $string=~/ ^ \Q$myvar\E REC: (.) /x )` [download] Update: Added links to docs. Also added bit about whitespace in the variable under `/x`. Update 2: Just to be clear, I recommend you always use `\Q...\E`, even if you don't use `/x`, unless you specifically want regex metacharacters in `$myvar` to be treated as such and it comes from a trusted source! That is, `if($string=~/^\Q$myvar\EREC:(.*)/)`	[reply] [d/l] [select]
Re: How do you use a variable value within a regular expression pattern properly? by 1nickt (Canon) on Sep 11, 2017 at 11:30 UTC
Use braces, like: `"${myvar}REC"`. The way forward always starts with a minimal test.	[reply] [d/l]
Re^2: How do you use a variable value within a regular expression pattern properly? by Anonymous Monk on Sep 11, 2017 at 11:38 UTC
Thank you very much!	[reply]