Regular expression (regex) in JavaScript

Question

I need a regex that can validate a string in the following criteria.

Beginning with the verbatim text ROLE_.
Only a combination of capital letters and undescores are allowed.
Must have minimum of 6 and maximum of 20 characters in it.
Two or more consecutive underscores are disallowed.
Leading and/or trailing underscores are disallowed.
The text ROLE_ADMIN is disallowed.

Accordingly, the following regex works in Java.

(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})

But in JavaScript, a string like ROLE_ADMIN_s (s at the end in small case) is treated as a valid string that shouldn't actually be. The maximum allowable characters (20) are also not validated correctly.

I'm using the match() function like,

if($("#txtAuthority").val().match("(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})"))
{
     //...
}

You need to add ^ to the start and $ to the end. Also, it's not your issue because the browser handles it automatically in this case, but you should wrap regexps in /myexpression/ instead of "myexpression" — Dave
– Dave, Commented May 5, 2013 at 16:18
:-) I had reverse trouble while moving from js to java - adding .* at start & end... — anishsane
– anishsane, Commented May 5, 2013 at 16:22

Gumbo · Accepted Answer · 2013-05-05 16:19:18Z

1

Java’s matches method expects the pattern to match the whole string while JavaScript’s match method is rather a find any match method as it suffices that the pattern is found somewhere inside the string.

"ROLE_ADMIN_s".match(/(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})/)[0] === "ROLE_ADMIN_"

If you want JavaScript’s match to act like Java’s matches, use anchors for the begin (^) and end ($) of the string:

/^(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})$/

This will fail on your string:

"ROLE_ADMIN_s".match(/^(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})$/) === null

answered May 5, 2013 at 16:19

Gumbo

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3 Comments

Tiny Over a year ago

/^(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})$/ evaluates to false on valid strings like - ROLE_ADMIN_S.

Tiny Over a year ago

This works when I remove the forward slash / from both sides of the expression like ^(?!.*__.*)(?!^ROLE_ADMIN$)(?!.*_$)(ROLE_[A-Z_]{1,15})$. I don't get it correctly.

Gumbo Over a year ago

@Tiny In JavaScript, RegExp objects have a literal syntax that is /<pattern>/<flags>. Of course, if you use those literals as a string, i.e. "/<pattern>/<flags>", the delimiters / are also treated as part of the pattern.

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