Match regex using array

Question

I made this in C++ and I wanted to convert to JavaScript:

foreach (QString pattern, extensions) {
    regex.setPattern(QString("\\.%1").arg(pattern));
    regex.setPatternOptions(QRegularExpression::CaseInsensitiveOption);

    QRegularExpressionMatch match = regex.match(filename);

    if (! match.hasMatch()) continue;

    return pattern;
}

It means that foreach extensions (that is an array of extensions) as pattern create a pattern with that to be like: \\.png (for example). If there's a match it will return the found extension.

I tried to create exactly how I did in C++ but I don't know how to concatenate the returned string from the array to match

const filename = 'example.wutt'
const extensions = ['wutt', 'xnss']

extensions.forEach(pattern => {
  const match = filename.match(`\\.${pattern}`)
  console.log(match)
})

It does work but it's not case-insensitive as I can't put the i flag.

How can I do that (and if there's a solution using ES6)?

you are missing a closing tick mark, and a closing parenthesis — Get Off My Lawn
– Get Off My Lawn, Commented Oct 12, 2017 at 16:06
I edited my question and I didn't noticed that I removed, but I put it back. — vajehu
– vajehu, Commented Oct 12, 2017 at 16:07
Also your c++ version returns a value in the loop, forEach in javascript won't break out of a forEach function if you return. — Get Off My Lawn
– Get Off My Lawn, Commented Oct 12, 2017 at 16:09
I didn't know that stackoverflow were this smart, I put the variable there as well — vajehu
– vajehu, Commented Oct 12, 2017 at 16:09

Felix Kling · Accepted Answer · 2017-10-12 16:24:53Z

3

Have a look at How do you use a variable in a regular expression? for building the regex.

If you want to find the extension that matches, you can use Array#find:

const matchedExtension = extensions.find(
  ext => new RegExp(String.raw`\.${ext}$`, 'i').test(filename)
);

var extensions = ['png', 'jpeg'];

var filename = 'foo.png';

console.log(extensions.find(
  ext => new RegExp(String.raw `\.${ext}$`, 'i').test(filename)
));

Couple of notes:

String.raw is necessary to not treat \. as a string escape sequence but to pass it "as is" to the regular expression engine (alternative you could escape the \, but String.raw is cool).
$ at the end of the pattern ensures that the pattern is only matched at the end of the file name.
If you just want to know whether a pattern matches or not, RegExp#test is the preferred method.

If you are doing this a lot it makes sense to generate an array of regular expressions first (instead of creating the regex every time you call the function).

edited Oct 12, 2017 at 16:24

answered Oct 12, 2017 at 16:10

Felix Kling

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3 Comments

guest271314 Over a year ago

Why use .find() instead of .filter()?

Felix Kling Over a year ago

From the question: "If there's a match it will return the found extension." .filter would work I guess, but there is probably only a single extension that matches.

vajehu Over a year ago

You rock! Thank you all <3

guest271314 · Accepted Answer · 2017-10-12 16:22:12Z

3

You can use RegExp constructor with "i" passed as second argument

extensions.forEach(pattern => {
    const match = filename.match(new RegExp(`\\.${pattern}$`, "i"));
    console.log(match);
})

edited Oct 12, 2017 at 16:22

answered Oct 12, 2017 at 16:08

guest271314

1

4 Comments

Felix Kling Over a year ago

Note that . matches any character. So If the filename was opngate.gif, it would match .png. You have to escape the . properly.

guest271314 Over a year ago

@FelixKling opngate.gif would not be matched using the pattern at Answer though you do have a point, see updated post.

Felix Kling Over a year ago

Run "opngate.gif".match(new RegExp(`.png`, "i")) in your console ;)

guest271314 Over a year ago

@FelixKling You are correct. Ran test improperly. Was also missing $ within RegExp. [.]${pattern} could alternatively be used.

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