Does JavaScript regex matching require repeated code?

Question

Consider the following regex code snippet in Perl:

if ($message =~ /^(.+) enters the race!$/)) {
    $racer = $1;
    print "Found $racer";
} else {
    print "Racer parsing error!";
}

I'm trying to port this to JavaScript, and here's what I have come up with:

if (message.match(/^.+ enters the race!$/)) {
    var racer = message.match(/^(.+) enters the race!$/)[1];
    console.log("Found " + racer);
} else {
    console.log("Racer parsing error!");
}

Notice how the regex has to be repeated twice. This look sloppy. Not to mention that it wastes processing power since it has to do the same regex twice in a row. Is there any way to make this code snippet look cleaner?

Might as well just do if (racer = message.match(/^(.+) enters the race!$/)[1] ) { ..} else { ..} — user557597
– user557597, Commented Mar 1, 2016 at 20:09

ak_ · Accepted Answer · 2016-03-01 19:56:39Z

2

You can check the regex match right in the if statement. Something like this would work:

JavaScript

function check(message) {
    if (racer = message.match(/^(.+) enters the race!$/)) {
    console.log("Found " + racer[1]);
    } else {
    console.log("Racer parsing error!");
    }
}


check("blah enters the race!")
check("blah blah blah")

Output

Found blah
Racer parsing error!

edited Mar 1, 2016 at 19:56

answered Mar 1, 2016 at 19:54

ak_

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1 Comment

ak_ Over a year ago

@WiktorStribiżew if the capture group is empty, there is no regex match and the else statement is triggered.

Wiktor Stribiżew · Accepted Answer · 2016-03-01 21:05:16Z

There are some differences between the code you have in Perl and JS:

JS does not have string interpolation, so your code has to be a bit verbose
There are no $1-like global variables, though you can declare it and use in code.

You can first match and check if the regex matched anything.

var message = "John enters the race!";
var m = message.match(/^(.+) enters the race!$/); // Match the string with a pattern
if (m) {                                          // Check if there is a match
    var racer = m[1];                             // Assign the captured substring to `racer`
    console.log("Found " + racer);              
} else {
    console.log("Racer parsing error!");
}

Note that m here is a Match object containing:

m[0] - the whole matched text
m[1] - the contents of Group 1 (capture group 1)
m.index - the 0-based index of the match in the string
m.input - the original string

Keith Hall · Accepted Answer · 2016-03-01 19:57:19Z

1

You can store the match result in a variable:

var result = message.match(/^.+ enters the race!$/);
if (result) {
    var racer = result[1];
    console.log("Found " + racer);
} else {
    console.log("Racer parsing error!");
}

answered Mar 1, 2016 at 19:57

Keith Hall

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Comments

jmcgriz · Accepted Answer · 2016-03-01 19:58:33Z

0

You can run the match command once. If it fails, getmatches will be null. If it's successful, getmatches[1] will contain your racer's name

var rgx = /^(.+) enters the race!$/,
  message = window.prompt('Enter Message'),
  getmatches = message.match(rgx);
if (getmatches) {
  var racer = getmatches[1];
  console.log("Found " + racer);
} else {
  console.log("Racer parsing error!");
}

answered Mar 1, 2016 at 19:58

jmcgriz

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Comments

leo.fcx · Accepted Answer · 2016-03-01 20:06:03Z

0

I think you should create a RegExp instance and use test() and exec() methods as follows:

var myRegExp = new RegExp('^(.+) enters the race!$');

var message = 'Someone enters the race!';

if (myRegExp.test(message)) {
  var racer = myRegExp.exec(message)[1];
  console.log('Found ' + racer);
} 
else {
  console.log('Racer parsing error!');
}

Note the usage of ( and ) in the regular expression.

answered Mar 1, 2016 at 20:06

leo.fcx

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