Checking strings for patterns in JavaScript

Write code deliberately, and pick the right tool for the job

JavaScript offers us several methods for checking strings for patterns, most notably found on RegExp and String  objects. This article aims to highlight two methods I see being used  for checking strings for patterns, how they might be misused, and how  you can use them appropriately.

RegExp.prototype.test()

String.prototype.match()

This article assumes you understand what regular expressions are, how to infer their meaning, and when it’s appropriate to use them.

Problem: Write  a function that takes a string and a pattern as its inputs and prints  “Pattern Found” if the pattern is found in the string and “Pattern Not  Found” otherwise.

Although  masked in simplicity and maybe even triviality, developers frequently  encounter problems similar to this one — a problem where we must check  that a string contains a certain pattern and write code that does one  thing when the pattern is found and take another course of action when  the pattern is absent from the string. We encounter this problem so  often that we develop a solution that sits ready to be pulled out of our  left brains at a moment’s notice.

If  you write JavaScript and encounter this problem, take a minute to think  about which one of the below solutions you are more likely to write.  Both solutions produce the same results in every case, but I propose  that one is better than the other for this problem.

Solution #1

function testStrForPattern(str, pattern) {

  if (str.match(pattern)) {

    return console.log("Pattern Found");

  }

  return console.log("Pattern Not Found");

}

const sentence = "We write JavaScript code for people!";

testStrForPattern(sentence, /javascript|code/i);

// Output: "Pattern Found"



testStrForPattern(sentence, /php|web/i);

// Output: "Pattern Not Found"

Solution #2

function testStrForPattern(str, pattern) {

  if (pattern.test(str)) {

    return console.log("Pattern Found");

  }

  return console.log("Pattern Not Found");

}

const sentence = "We write JavaScript code for people!";

testStrForPattern(sentence, /javascript|code|web/i);

// Output: "Pattern Found"



testStrForPattern(sentence, /php|web/i);

// Output: "Pattern Not Found"

Pick the right tool for the job

“Pick  the right tool for the job” is a sentence we hear a lot in industry,  and I suspect many of us really try to abide. I hear this sentence most  frequently used when addressing the macro solution to a problem, for  example, in using an appropriate software package or in choosing an  appropriate software development methodology. Now, I am making the same  recommendation to pick the right tool for the job at the micro level (in  code), because it matters there too.

String.prototype.match()

str.match(pattern)

If the string matches the expression, it will return an Array  containing the entire matched string as the first element, followed by  any results captured in parentheses. If there were no matches, null is returned.

RegExp.prototype.test()

pattern.test(str)

Returns true if there is a match between the regular expression and the specified string; otherwise, false.

The first solution works because if the pattern is found, the returned array will evaluate to true in the if statement and false when null is return due to the absence on the pattern in the string.

However, match was not designed to simply and efficiently check if a pattern is found in a string. Anticipating the misuse of match, the Mozilla Developer Network documents,

If you need to know if a string matches a regular expression RegExp, use RegExp.test()

Finally, we can intuit that match performs slower than test.  Just imagine the extra work being done to build up an array of all the  matched substrings as opposed to returning a single value on the first  occurrence of a match. Still don’t believe me? Take a look at how much  faster test is than match here. It’s much faster!

Final words

Use test to do a simple boolean check for whether or not a pattern exists in a string and match  to build up an array of the matches found in the string. When it comes  to solving problems with code, picking the right tool for the job is  just as important in the micro as it is in the macro. 

 Originally posted on my blog at medium.com.