Looking for a regular expression to validate input, specifically filtering out all special characters except "underscore". All characters within the range [a-zA-Z0-9\underscore] are permitted and can appear multiple times. However, my expression should not allow 2 consecutive occurrences of "underscore" or have the string start with "_".
To ensure that the repetitions work effectively, I suggest utilizing groups in your regular expression pattern:
/[A-Za-z0-9]+(?:_(?:[A-Za-z0-9]+|$))*/
/^([a-z0-9]_?)+$/
At least one... letter or number, with the option of adding an underscore at the end
There are times when flipping your perspective—looking for any issues instead of ensuring there are none—can lead to a more straightforward solution:
// checks if it starts with underscore
// or has two underscores in a row
// or includes a non-alphanumeric/underscore character
var invalid = /^_|_{2}|\W/;
if (invalid.test(input)) handleInvalidInput();
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