{"id":187,"date":"2010-02-13T16:20:19","date_gmt":"2010-02-13T23:20:19","guid":{"rendered":"http:\/\/kai.mactane.org\/blog\/?p=187"},"modified":"2011-07-04T10:27:44","modified_gmt":"2011-07-04T17:27:44","slug":"silly-coding-tricks-inverted-string-match","status":"publish","type":"post","link":"https:\/\/kagan.mactane.org\/blog\/2010\/02\/13\/silly-coding-tricks-inverted-string-match\/","title":{"rendered":"Silly Coding Tricks: “Inverted” String Match"},"content":{"rendered":"

First things first: Never actually do this.<\/strong> This is just a fun curiosity, for amusement value only.<\/p>\n

Because of the way JavaScript’s search()<\/code> method works, you can do:<\/p>\n

var my_url = 'http:\/\/kai.mactane.org';
\nif (! my_url.search(\/http:\\\/\\\/kai.mactane.org\/)) {
\n    alert("Your URL is http:\/\/kai.mactane.org");
\n} else {
\n    alert("Nope, your URL isn't http:\/\/kai.mactane.org");
\n}<\/div>\n

Try running this, and it will correctly claim that “Your URL is http:\/\/kai.mactane.org” — even though there’s a ! at the beginning of the test. What gives?<\/p>\n

That exclamation point looks<\/em> like it’s saying: “If the search for http:\/\/kai.mactane.org<\/code> within the given string fails…” But what that test is really<\/em> saying is: “If the search for http:\/\/kai.mactane.org<\/code> returns Boolean false — or anything that evaluates as<\/em> Boolean false…”<\/p>\n

What that search()<\/code> call actually returns is the character index<\/em> at which the needle is found, or -1 if it isn’t found. Since the needle is found at the very beginning of the haystack — at character index 0 — search()<\/code> returns zero. Which evaluates to false in a Boolean context.<\/p>\n

If the needle isn’t found, the search will return -1 — which evaluates as true<\/em> in a Boolean context!<\/p>\n

Effectively, the ! operator is reconciling the disagreement between search()<\/code>’s idea of falsehood and if()<\/code>’s idea of falsehood: search()<\/code> returns -1 for a false result (a failed match), but if()<\/code> considers -1 to be true, not false.<\/p>\n

This trick only works with needles that you are sure will only occur at the beginning of the haystack.<\/p>\n

Once again, you should never, ever actually<\/strong> write code like this<\/em> for any serious purpose or in anything you intend to deploy in the real world. The maintainability problems are not worth the amusement of confusing all your co-workers with a test that looks reversed. Instead, use a positional anchor in your regex, and explicitly test against -1…<\/p>\n

if (my_url.search(\/^http:\\\/\\\/kai.mactane.org) != -1)<\/div>\n

…like any sensible JavaScript coder. (Yes, that last bit is intended to be ironic. I wrote something once about the silliness of having to add that extra test; I’ll have to see if I can find it and republish it here. )<\/p>\n","protected":false},"excerpt":{"rendered":"

First things first: Never actually do this. This is just a fun curiosity, for amusement value only. Because of the way JavaScript’s search() method works, you can do: var my_url = 'http:\/\/kai.mactane.org'; if (! my_url.search(\/http:\\\/\\\/kai.mactane.org\/)) {     alert("Your URL is http:\/\/kai.mactane.org"); } else {     alert("Nope, your URL isn't http:\/\/kai.mactane.org"); } Try running this, and it will […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[63,7,83,82],"_links":{"self":[{"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/posts\/187"}],"collection":[{"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/comments?post=187"}],"version-history":[{"count":7,"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/posts\/187\/revisions"}],"predecessor-version":[{"id":346,"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/posts\/187\/revisions\/346"}],"wp:attachment":[{"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/media?parent=187"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/categories?post=187"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kagan.mactane.org\/blog\/wp-json\/wp\/v2\/tags?post=187"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}