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How to write regexp for finding :smile: in javascript?

I want to write a regular expression, in JavaScript, for finding the string starting and ending with :.
For example "hello :smile: :sleeping:" from this string I need to find the strings which are starting and ending with the : characters. I tried the expression below, but it didn't work:
^:.*\:$

My guess is that you not only want to find the string, but also replace it. For that you should look at using a capture in the regexp combined with a replacement function.
const emojiPattern = /:(\w+):/g
function replaceEmojiTags(text) {
return text.replace(emojiPattern, function (tag, emotion) {
// The emotion will be the captured word between your tags,
// so either "sleep" or "sleeping" in your example
//
// In this function you would take that emotion and return
// whatever you want based on the input parameter and the
// whole tag would be replaced
//
// As an example, let's say you had a bunch of GIF images
// for the different emotions:
return '<img src="/img/emoji/' + emotion + '.gif" />';
});
}
With that code you could then run your function on any input string and replace the tags to get the HTML for the actual images in them. As in your example:
replaceEmojiTags('hello :smile: :sleeping:')
// 'hello <img src="/img/emoji/smile.gif" /> <img src="/img/emoji/sleeping.gif" />'
EDIT: To support hyphens within the emotion, as in "big-smile", the pattern needs to be changed since it is only looking for word characters. For this there is probably also a restriction such that the hyphen must join two words so that it shouldn't accept "-big-smile" or "big-smile-". For that you need to change the pattern to:
const emojiPattern = /:(\w+(-\w+)*):/g
That pattern is looking for any word that is then followed by zero or more instances of a hyphen followed by a word. It would match any of the following: "smile", "big-smile", "big-smile-bigger".

The ^ and $ are anchors (start and end respectively). These cause your regex to explicitly match an entire string which starts with : has anything between it and ends with :.
If you want to match characters within a string you can remove the anchors.
Your * indicates zero or more so you'll be matching :: as well. It'll be better to change this to + which means one or more. In fact if you're just looking for text you may want to use a range [a-z0-9] with a case insensitive modifier.
If we put it all together we'll have regex like this /:([a-z0-9]+):/gmi
match a string beginning with : with any alphanumeric character one or more times ending in : with the modifiers g globally, m multi-line and i case insensitive for things like :FacePalm:.
Using it in JavaScript we can end up with:
var mytext = 'Hello :smile: and jolly :wave:';
var matches = mytext.match(/:([a-z0-9]+):/gmi);
// matches = [':smile:', ':wave:'];
You'll have an array with each match found.

Replace words of text area

I have made a javascript function to replace some words with other words in a text area, but it doesn't work. I have made this:
function wordCheck() {
var text = document.getElementById("eC").value;
var newText = text.replace(/hello/g, '<b>hello</b>');
document.getElementById("eC").innerText = newText;
}
When I alert the variable newText, the console says that the variable doesn't exist.
Can anyone help me?
Edit:
Now it replace the words, but it replaces it with <b>hello</b>, but I want to have it bold. Is there a solution?

Update:
In response to your edit, about your wanting to see the word "hello" show up in bold. The short answer to that is: it can't be done. Not in a simple textarea, at least. You're probably looking for something more like an online WYSIWYG editor, or at least a RTE (Richt Text Editor). There are a couple of them out there, like tinyMCE, for example, which is a decent WYSIWYG editor. A list of RTE's and HTML editors can be found here.
First off: As others have already pointed out: a textarea element's contents is available through its value property, not the innerText. You get the contents alright, but you're trying to update it through the wrong property: use value in both cases.
If you want to replace all occurrences of a string/word/substring, you'll have to resort to using a regular expression, using the g modifier. I'd also recommend making the matching case-insensitive, to replace "hello", "Hello" and "HELLO" all the same:
var txtArea = document.querySelector('#eC');
txtArea.value = txtArea.value.replace(/(hello)/gi, '<b>$1</b>');
As you can see: I captured the match, and used it in the replacement string, to preserve the caps the user might have used.
But wait, there's more:
What if, for some reason, the input already contains <b>Hello</b>, or contains a word containing the string "hello" like "The company is called hellonearth?" Enter conditional matches (aka lookaround assertions) and word boundaries:
txtArea.value = txtArea.value.replace(x.value.replace(/(?!>)\b(hello)\b(?!<)/gi, '<b>$1</b>');
fiddle
How it works:
(?!>): Only match the rest if it isn't preceded by a > char (be more specific, if you want to and use (?!<b>). This is called a negative look-ahead
\b: a word boundary, to make sure we're not matching part of a word
(hello): match and capture the string literal, provided (as explained above) it is not preceded by a > and there is a word boundary
(?!<): same as above, only now we don't want to find a matching </b>, so you can replace this with the more specific (?!<\/b>)
/gi: modifiers, or flags, that affect the entire pattern: g for global (meaning this pattern will be applied to the entire string, not just a single match). The i tells the regex engine the pattern is case-insensitive, ie: h matches both the upper and lowercase character.
The replacement string <b>$1</b>: when the replacement string contains $n substrings, where n is a number, they are treated as backreferences. A regex can group matches into various parts, each group has a number, starting with 1, depending on how many groups you have. We're only grouping one part of the pattern, but suppose we wrote:
'foobar hello foobar'.replace(/(hel)(lo)/g, '<b>$1-$2</b>');
The output would be "foobar <b>hel-lo</b> foobar", because we've split the match up into 2 parts, and added a dash in the replacement string.
I think I'll leave the introduction to RegExp at that... even though we've only scratched the surface, I think it's quite clear now just how powerful regex's can be. Put some time and effort into learning more about this fantastic tool, it is well worth it.

If <textarea>, then you need to use .value property.
document.getElementById("eC").value = newText;
And, as mentioned Barmar, replace() replaces only first word. To replace all word, you need to use simple regex. Note that I removed quotes. /g means global replace.
var newText = text.replace(/hello/g, '<b>hello</b>');
But if you want to really bold your text, you need to use content editable div, not text area:
<div id="eC" contenteditable></div>
So then you need to access innerHTML:
function wordCheck() {
var text = document.getElementById("eC").innerHTML;
var newText = text.replace(/hello/g, '<b>hello</b>');
newText = newText.replace(/<b><b>/g,"<b>");//These two lines are there to prevent <b><b>hello</b></b>
newText = newText.replace(/<\/b><\/b>/g,"</b>");
document.getElementById("eC").innerHTML = newText;
}

Regex trying to match characters before and after symbol

I'm trying to match characters before and after a symbol, in a string.
string: budgets-closed
To match the characters before the sign -, I do: ^[a-z]+
And to match the other characters, I try: \-(\w+) but, the problem is that my result is: -closed instead of closed.
Any ideas, how to fix it?
Update
This is the piece of code, where I was trying to apply the regex http://jsfiddle.net/trDFh/1/
I repeat: It's not that I don't want to use split; it's just I was really curious, and wanted to see, how can it be done the regex way. Hacking into things spirit
Update2
Well, using substring is a solution as well: http://jsfiddle.net/trDFh/2/ and is the one I chosed to use, since the if in question, is actually an else if in a more complex if syntax, and the chosen solutions seems to be the most fitted for now.

Use exec():
var result=/([^-]+)-([^-]+)/.exec(string);
result is an array, with result[1] being the first captured string and result[2] being the second captured string.
Live demo: http://jsfiddle.net/Pqntk/

I think you'll have to match that. You can use grouping to get what you need, though.
var str = 'budgets-closed';
var matches = str.match( /([a-z]+)-([a-z]+)/ );
var before = matches[1];
var after = matches[2];
For that specific string, you could also use
var str = 'budgets-closed';
var before = str.match( /^\b[a-z]+/ )[0];
var after = str.match( /\b[a-z]+$/ )[0];
I'm sure there are better ways, but the above methods do work.

If the symbol is specifically -, then this should work:
\b([^-]+)-([^-]+)\b
You match a boundry, any "not -" characters, a - and then more "not -" characters until the next word boundry.
Also, there is no need to escape a hyphen, it only holds special properties when between two other characters inside a character class.
edit: And here is a jsfiddle that demonstrates it does work.

Javascript regex expression to replace multiple strings?

I've a string done like this: "http://something.org/dom/My_happy_dog_%28is%29cool!"
How can I remove all the initial domain, the multiple underscore and the percentage stuff?
For now I'm just doing some multiple replace, like
str = str.replace("http://something.org/dom/","");
str = str.replace("_%28"," ");
and go on, but it's really ugly.. any help?
Thanks!
EDIT:
the exact input would be "My happy dog is cool!" so I would like to get rid of the initial address and remove the underscores and percentage and put the spaces in the right place!
The problem is that trying to put a regex on Chrome "something goes wrong". Is it a problem of Chrome or my regex?

I'd suggest:
var str = "http://something.org/dom/My_happy_dog_%28is%29cool!";
str.substring(str.lastIndexOf('/')+1).replace(/(_)|(%\d{2,})/g,' ');
JS Fiddle demo.
The reason I took this approach is that RegEx is fairly expensive, and is often tricky to fine tune to the point where edge-cases become less troublesome; so I opted to use simple string manipulation to reduce the RegEx work.
Effectively the above creates a substring of the given str variable, from the index point of the lastIndexOf('/') (which does exactly what you'd expect) and adding 1 to that so the substring is from the point after the / not before it.
The regex: (_) matches the underscores, the | just serves as an or operator and the (%\d{2,}) serves to match digit characters that occur twice in succession and follow a % sign.
The parentheses surrounding each part of the regex around the |, serve to identify matching groups, which are used to identify what parts should be replaced by the ' ' (single-space) string in the second of the arguments passed to replace().
References:
lastIndexOf().
replace().
substring().

You can use unescape to decode the percentages:
str = unescape("http://something.org/dom/My_happy_dog_%28is%29cool!")
str = str.replace("http://something.org/dom/","");

Maybe you could use a regular expression to pull out what you need, rather than getting rid of what you don't want. What is it you are trying to keep?
You can also chain them together as in:
str.replace("http://something.org/dom/", "").replace("something else", "");

You haven't defined the problem very exactly. To get rid of all stretches of characters ending in %<digit><digit> you'd say
var re = /.*%\d\d/g;
var str = str.replace(re, "");

ok, if you want to replace all that stuff I think that you would need something like this:
/(http:\/\/.*\.[a-z]{3}\/.*\/)|(\%[a-z0-9][a-z0-9])|_/g
test
var string = "http://something.org/dom/My_happy_dog_%28is%29cool!";
string = string.replace(/(http:\/\/.*\.[a-z]{3}\/.*\/)|(\%[a-z0-9][a-z0-9])|_/g,"");

Create a permalink with JavaScript

I have a textbox where a user puts a string like this:
"hello world! I think that __i__ am awesome (yes I am!)"
I need to create a correct URL like this:
hello-world-i-think-that-i-am-awesome-yes-i-am
How can it be done using regular expressions?
Also, is it possible to do it with Greek (for example)?
"Γεια σου κόσμε"
turns to
geia-sou-kosme
In other programming languages (Python/Ruby) I am using a translation array. Should I do the same here?

Try this:
function doDashes(str) {
var re = /[^a-z0-9]+/gi; // global and case insensitive matching of non-char/non-numeric
var re2 = /^-*|-*$/g; // get rid of any leading/trailing dashes
str = str.replace(re, '-'); // perform the 1st regexp
return str.replace(re2, '').toLowerCase(); // ..aaand the second + return lowercased result
}
console.log(doDashes("hello world! I think that __i__ am awesome (yes I am!)"));
// => hello-world-I-think-that-i-am-awesome-yes-I-am
As for the greek characters, yeah I can't think of anything else than some sort of lookup table used by another regexp.
Edit, here's the oneliner version:
Edit, added toLowerCase():
Edit, embarrassing fix to the trailing regexp:
function doDashes2(str) {
return str.replace(/[^a-z0-9]+/gi, '-').replace(/^-*|-*$/g, '').toLowerCase();
}

A simple regex for doing this job is matching all "non-word" characters, and replace them with a -. But before matching this regex, convert the string to lowercase. This alone is not fool proof, since a dash on the end may be possible.
[^a-z]+
Thus, after the replacement; you can trim the dashes (from the front and the back) using this regex:
^-+|-+$
You'd have to create greek-to-latin glyps translation yourself, regex can't help you there. Using a translation array is a good idea.

I can't really say for Greek characters, but for the first example, a simple:
/[^a-zA-Z]+/
Will do the trick when using it as your pattern, and replacing the matches with a "-"
As per the Greek characters, I'd suggest using an array with all the "character translations", and then adding it's values to the regular expression.

To roughly build the url you would need something like this.
var textbox = "hello world! I think that __i__ am awesome (yes I am!)";
var url = textbox.toLowerCase().replace(/([^a-z])/, '').replace(/\s+/, " ").replace(/\s/, '-');
It simply removes all non-alpha characters, removes double spacing, and then replaces all space chars with a dash.
You could use another regular expression to replace the greek characters with english characters.