BaseHtml::escapeJsRegularExpression() fails with non-slash delimiters and complex patterns

[santilin]

The function yii\helpers\BaseHtml::escapeJsRegularExpression() fails to process valid regular expressions when the delimiter is not /. For example, it fails with this input:

([a-z_])([a-z0-9_])*

Expected output:

/([a-z_])([a-z0-9_])*/

Actual output:

/[a-z_])/

The function truncates the input because it assumes the first character is a delimiter and tries to find the last occurrence of it to extract the pattern. This breaks when the pattern contains the first character in the middle of the pattern.