When Not to Use Regular Expressions When used with care, regexes are very fast. However, they’re usually overkill when you are merely searching for literal strings. This is especially true if you know in advance which part of a string you want to test. For instance, if you want to check whether a string ends with a semicolon, you could use something like this: endsWithSemicolon = /;$/.test(str);
You might be surprised to learn, though, that none of the big browsers are currently smart enough to realize in advance that this regex can match only at the end of the string. What they end up doing is stepping through the entire string. Each time a semicolon is found, the regex advances to the next token ($), which checks whether the match is at the end of the string. If not, the regex continues searching for a match until it finally makes its way through the entire string. The longer your string (and the more semicolons it contains), the longer this takes. In this case, a better approach is to skip all the intermediate steps required by a regex and simply check whether the last character is a semicolon: endsWithSemicolon = str.charAt(str.length - 1) == ";";
This is just a bit faster than the regex-based test with small target strings, but, more importantly, the string’s length no longer affects the time needed to perform the test. This example used the charAt method to read the character at a specific position. The string methods slice, substr, and substring work well when you want to extract and check the value of more than one character at a specific position. Additionally, the indexOf and lastIndexOf methods are great for finding the position of literal strings or checking for their presence. All of these string methods are fast and can help you avoid invoking the overhead of regular expressions when searching for literal strings that don’t rely on fancy regex features.
String Trimming Removing leading and trailing whitespace from a string is a simple but common task. Although ECMAScript 5 adds a native string trim method (and you should therefore start to see this method in upcoming browsers), JavaScript has not historically included it. For the current browser crop, it’s still necessary to implement a trim method yourself or rely on a library that includes it. Trimming strings is not a common performance bottleneck, but it serves as a decent case study for regex optimization since there are a variety of ways to implement it.
Trimming with Regular Expressions Regular expressions allow you to implement a trim method with very little code, which is important for JavaScript libraries that focus on file size. Probably the best all-around String Trimming | 99