Could a Javascript regex error be resolved with the addition of look-behind functionality?

Question

Could a Javascript regex error be resolved with the addition of look-behind functionality?

Currently, I am working on an exercise within a Javascript learning platform.

The task is as follows:

Input: A string consisting of words, with some words possibly containing hashtags (prefixed with #).
Output: An array of strings that have been prefixed with a hashtag but do not contain the actual hashtag character.
It's important to note that single pound signs alone are not considered valid inputs (e.g., "#" would result in an empty array).
If a word has multiple leading hashtags, only the last one should be considered (e.g., "##example" would return ["example"])
Hashtags must not appear within the middle of a word (e.g., "in#line hashtag" should return an empty array)
Furthermore, hashtags must come before alphabetical characters (e.g., "#123abc" or "#?" are invalid inputs)

This was my attempt at solving the problem:

function getHashtags(post) {
  return /#(\w+)/.exec(post)
}

However, the output I'm receiving looks like this:

String Input: Hello #world
Outpu t: [ '#world', 'world', index: 6, input: 'Hello #world' ]

String Input: #lol #sorryNotSorry #heya #coolbeans
Output: [ '#lol','lol', index: 0, input: '#lol #sorryNotSorry #heya #coolbeans']

String Input: # # # #
Output: null

String Input: this is an in#line hash
Output: [ '#line', 'line', index: 13, input: 'this is an in#line hash' ]

String Input: too ##many tags
Output: [ '#many', 'many', index: 5, input: 'too ##many tags' ]

String Input: invalid chars #$? #;wha
Output: null

String Input: "" //empty string
null

String Input: #blue#red#yellow#green
Output:[ '#blue', 'blue', index: 0, input: '#blue#red#yellow#green' ]

I believe I may need to implement lookback functionality, although it's not directly supported in Javascript and I haven't found a suitable workaround yet. Can anyone offer any guidance?

javascript arrays regex string

Answer 1

Answer №1

It seems like you're on the right track with your approach. Using exec will give you the first set of results, and by continuing to call it with the global flag g, you can retrieve subsequent matches. The example provided here is sourced from mozilla's website:

var myRe = /ab*/g;
var str = 'abbcdefabh';
var myArray;
while ((myArray = myRe.exec(str)) !== null) {
  var msg = 'Found ' + myArray[0] + '. ';
  msg += 'Next match starts at ' + myRe.lastIndex;
  console.log(msg);
}

Exec

I must commend you for presenting your question in a clear manner that demonstrates your attempts at solving the issue. Now, let me guide you through how you could implement this solution:

function getHashtags(post)
{
    regex = /#(\w+)/g;
    arr = [];

    while((results = regex.exec(post)) !== null)
    {
        arr.push(results[1]);
    }

    return arr;
}

Answer 2

It seems like you're on the right track with your approach. Using exec will give you the first set of results, and by continuing to call it with the global flag g, you can retrieve subsequent matches. The example provided here is sourced from mozilla's website:

var myRe = /ab*/g;
var str = 'abbcdefabh';
var myArray;
while ((myArray = myRe.exec(str)) !== null) {
  var msg = 'Found ' + myArray[0] + '. ';
  msg += 'Next match starts at ' + myRe.lastIndex;
  console.log(msg);
}

Exec

I must commend you for presenting your question in a clear manner that demonstrates your attempts at solving the issue. Now, let me guide you through how you could implement this solution:

function getHashtags(post)
{
    regex = /#(\w+)/g;
    arr = [];

    while((results = regex.exec(post)) !== null)
    {
        arr.push(results[1]);
    }

    return arr;
}

Answer 3

Answer №2

Do not place hashtags in the middle of a word (e.g. "in#line hashtag" results in an empty array)

-- Use the non-word boundary code \B to prevent a word character from appearing right before the #. Additionally, to avoid matching when the # is within the hashtag, include a word boundary that is not followed by #: (?!#)\b.

Hashtags should come before alphabetical characters (e.g. "#120398" or "#?" are not valid)

-- Add [a-zA-Z] immediately after # and then utilize \w. If using /i modifier, use [a-z].

Therefore, use

/\B#+([a-z]\w*(?!#)\b)/gi

Check out the demo

This method caters to basic Latin-script based hashtag extraction.

function getHashtags(post) {
  var re = /\B#+([a-z]\w*(?!#)\b)/gi;
  arr = []; 
  while ((m = re.exec(post)) !== null) {
    arr.push(m[1]);
    document.write("Hashtag: " + m[0] + ", name: " + m[1] + "<br/>");
  }
  return arr;
}


var strs = ['##alot', 'Hello #world', '#lol #sorryNotSorry #heya #coolbeans', '# # # #', 'this is an in#line hash', 'too ##many tags', 'invalid chars #$? #;wha', '', '#blue#red#yellow#green'];
strs.forEach(function (str) {
  console.log(getHashtags(str));
});

Answer 4

Do not place hashtags in the middle of a word (e.g. "in#line hashtag" results in an empty array)

-- Use the non-word boundary code \B to prevent a word character from appearing right before the #. Additionally, to avoid matching when the # is within the hashtag, include a word boundary that is not followed by #: (?!#)\b.

Hashtags should come before alphabetical characters (e.g. "#120398" or "#?" are not valid)

-- Add [a-zA-Z] immediately after # and then utilize \w. If using /i modifier, use [a-z].

Therefore, use

/\B#+([a-z]\w*(?!#)\b)/gi

Check out the demo

This method caters to basic Latin-script based hashtag extraction.

function getHashtags(post) {
  var re = /\B#+([a-z]\w*(?!#)\b)/gi;
  arr = []; 
  while ((m = re.exec(post)) !== null) {
    arr.push(m[1]);
    document.write("Hashtag: " + m[0] + ", name: " + m[1] + "<br/>");
  }
  return arr;
}


var strs = ['##alot', 'Hello #world', '#lol #sorryNotSorry #heya #coolbeans', '# # # #', 'this is an in#line hash', 'too ##many tags', 'invalid chars #$? #;wha', '', '#blue#red#yellow#green'];
strs.forEach(function (str) {
  console.log(getHashtags(str));
});

Could a Javascript regex error be resolved with the addition of look-behind functionality?

Answer №1

Answer №2

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