Javascript regex returning true.. then false.. then true.. etc [duplicate]

by 
/^[^-_]([a-z0-9-_]{4,20})[^-_]$/gi;

You’re using a g (global) RegExp. In JavaScript, global regexen have state: you call them (with exec, test etc.) the first time, you get the first match in a given string. Call them again and you get the next match, and so on until you get no match and it resets to the start of the next string. You can also write regex.lastIndex= 0 to reset this state.

(This is of course an absolutely terrible piece of design, guaranteed to confuse and cause weird errors. Welcome to JavaScript!)

You can omit the g from your RegExp, since you’re only testing for one match.

Also, I don’t think you want [^-_] at the front and back. That will allow any character at each end, ie. *plop! would be valid. You’re probably thinking of lookahead/lookbehind assertions, but they’re not available in JavaScript. (Well, lookahead is supposed to be, but it’s broken in IE.) Suggest instead:

/^[a-z0-9][a-z0-9_-]{2,18}[a-z0-9]$/i

More Related Contents:

  1. How to send data to the server via Ajax?
  2. HTML: how refresh information in [closed]
  3. How to make page loaders/animations/transitions
  4. How to send FormData objects with Ajax-requests in jQuery? [duplicate]
  5. Refresh/reload the content in Div using jquery/ajax
  6. Set timeout for ajax (jQuery)
  7. Using HTML5 file uploads with AJAX and jQuery
  8. Prevent Page Load on Jquery Form Submit with None Display Button
  9. Why is it a bad practice to return generated HTML instead of JSON? Or is it?
  10. “Mixed content blocked” when running an HTTP AJAX operation in an HTTPS page
  11. How to bind Events on Ajax loaded Content?
  12. jquery submit form and then show results in an existing div
  13. How do I abort image load requests without using window.stop()
  14. how to assign javascript variable value to php variable
  15. How to generate a thumbnail image after adding an image inside an input type=”file” in a form and submitting them both on the same form
  16. How to replace html element with ajax response?
  17. How to change URL in browser without navigating away from page?
  18. How can I get all a form’s values that would be submitted without submitting [duplicate]
  19. When loading an html page via ajax, will script tags be loaded?
  20. how to use jQuery ajax calls with node.js
  21. jQuery DataTables “No Data Available in Table”
  22. Twitter-style autocomplete in textarea
  23. Scrolling Overflowed DIVs with JavaScript
  24. Remove everything within script and style tags
  25. jquery ajax parse response text
  26. Parsing returned HTML from jQuery AJAX request
  27. How to replace the entire html webpage with ajax response?
  28. Find and replace nth occurrence of [bracketed] expression in string
  29. How to silently hide “Image not found” icon when src source image is not found
  30. Anchor tag download attribute not working :Bug in Chrome 35.0.1916.114

Leave a Comment