Regex on a variable with special characters

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

I have 2 variables one is simple like below.

$x = "SERVER-NAME";
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and I am trying to see if $y starts with $x, but $y sometimes contains regex special characters like ()[], also ** which can give me errors.

How can I safely check if $x at the start of $y?

$y =~ /^$x/;
[download]

is the current regex I am using.

Comment on Regex on a variable with special characters Select or Download Code

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Re: Regex on a variable with special characters by haukex (Archbishop) on Jul 22, 2016 at 08:47 UTC
Hi jaydee, `$y =~ /^\Q$x\E/;` [download] See also quotemeta and Mind the meta! Update: Actually, you said "$y sometimes contains regex special characters" - my answer applies if $x contains special characters. If $y contains special characters and $x does not, you don't necessarily need to escape $x, although the general advice in Mind the meta! still applies that it's safer to always use `\Q...\E` except in the case that you intentionally want the contents of the variable being interpolated into the regex to be interpreted as a regex. Update 2: Clarified wording. Hope this helps, -- Hauke D	[reply] [d/l] [select]
Re^2: Regex on a variable with special characters by jaydee (Initiate) on Jul 22, 2016 at 09:02 UTC
This is exactly was I was looking for. Thank you.	[reply]
Re^2: Regex on a variable with special characters by Linicks (Scribe) on Jul 23, 2016 at 17:31 UTC
Interesting - sometimes something easy becomes hard. I rememeber years ago (as a sysadmin) when upgrading the network machines, one machine couldn't get an IP address from the DHCP server (all MS stuff, of course). It turned out there was a bug in the MS stack in such that machine called 'stores_01' failed - the underscore fubarred it. So, keep the names sane, then the code can remain sane ~ and easy Nick	[reply]
Re: Regex on a variable with special characters by hippo (Archbishop) on Jul 22, 2016 at 09:12 UTC
I am trying to see if $y starts with $x, but $y sometimes contains regex special characters ... Then you have no problem - your current regex is fine. It is only if `$x` has regex special characters that you need to worry. If `$x` is just a literal string rather than a regular expression per se, consider using index instead. Less error-prone and (usually) faster too.	[reply] [d/l] [select]
Re: Regex on a variable with special characters by Anonymous Monk on Jul 22, 2016 at 21:54 UTC
It seems to me that your regex is useless even if it doesn't contain metacharacters.	[reply]
Re^2: Regex on a variable with special characters by Anonymous Monk on Jul 23, 2016 at 23:21 UTC
I'm assuming you're trolling because the regex is not useless.	[reply]
Re^3: Regex on a variable with special characters by Anonymous Monk on Jul 24, 2016 at 10:18 UTC
I assumed the code he gave was a small but working program, but i didn't read correctly.	[reply]