Why Does My Regular Expression Pattern Fail with the \'v\' Flag in HTML Pattern Attribute?

Validity of Regular Expression Patterns with Unicode Flags

Issues with RegExp Patterns in HTML Pattern Attribute

When using a regular expression pattern with the 'v' flag in an HTML pattern attribute, you may encounter a "SyntaxError" error. This error occurs because the 'v' flag is automatically applied when compiling the pattern into a RegExp object.

Character Class Subtraction and Escaping

The 'v' flag introduces stricter escaping rules, which do not allow a literal '-' character at the end of a character class. This is in contrast to the 'u' flag, where there is no such restriction.

Regex Pattern with 'u' and 'v' Flags

Consider the following regex pattern:

^[a-zA-Z0-9+_.-]+@[a-zA-Z0-9]+\.[a-zA-Z0-9]+$

With the 'u' flag applied:

<code class="js">console.log(/^[a-zA-Z0-9+_.-]+@[a-zA-Z0-9]+\.[a-zA-Z0-9]+$/u.test("[email protected]")); // true</code>

With the 'v' flag applied (automatically in HTML pattern attribute):

<code class="js">console.log(/^[a-zA-Z0-9+_.\-]+@[a-zA-Z0-9]+\.[a-zA-Z0-9]+$/v.test("[email protected]")); // SyntaxError</code>

Resolution

To resolve the error, you must escape the '-' character at the end of the character class. Here's the corrected pattern:

^[a-zA-Z0-9+_.-]+@[a-zA-Z0-9]+\.[a-zA-Z0-9]+$

This pattern should now work correctly both with and without the 'v' flag.

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