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Is there way to change a substring in js? [closed]

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I have a string like:
def definition():
I want to change word def (for example), every instance of word def but not the "def"s that are part of other words
like this
console.log("def definition():".specialReplace("def", "abc"));
and result should be
abc definition():
not
abc abcinition():

Use String#replace or String#replaceAll with a regular expression:
const specialReplace = (str) => str.replaceAll(/\bdef\b/g, 'abc')
console.log(specialReplace("def definition")) // abc definition
console.log(specialReplace("def definition def")) // abc definition abc
In the regular expression, \b is a boundary type assertion that matches any word boundary, such as between a letter and a space.
Note that the same sequence \b is also used inside character class regular expression positions ([\b]), to match the backspace character.

How to use variables with double quotes into Regular Exception? [closed]

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I want to replace the "3" into "5". When it is using static it's working fine but when I use it through variable var allvar= '"3"'; it's not working fine.
Here is the jsfiddle link

new RegExp( /[allvar]+/g ); will construct a regular expression matching all uninterrupted sequences of one or more characters from the set a, l, v, a, r.
To construct a regular expression from a variable, you can do this:
new RegExp(allvar, 'g')
It would also be good to escape characters with special meaning to RegExp, unless you intend for allvar to contain regexp source. Unfortunately, RegExp.escape is still not in the language, so one would use a workaround.
new RegExp(escapeRegExp(allvar), 'g')

Regx for correct filename 1201_17-11-2015.zip [closed]

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I need to check whether a given file name is in the correct format or not.
Means:
first four numbers_two numbers-two numbers-4 numbers.zip
for that I need a regular expression.
Example file name is (1201_17-11-2015.zip) in javascript

var re = new RegExp('^\d{4}_\d{2}-\d{2}-\d{4}.zip$');
if (filename.match(re)) {
//successful match
}

You regexp could look something like this:
^\d{4}_\d\d-\d\d-\d{4}.zip$
^ is the beginning of your pattern
\d means any number
{n} means that the last pattern has to exist n-time
$ is the end of your pattern
On this site you can start learning how to use regular expression
Here you can test if your own regular expression is working...

I find it best to test scenarios at RegExr. None the less, what you're asking for is basic:
var result = "1201_17-11-2015.zip".match(/\d{4}_\d{2}-\d{2}-\d{4}\.zip/)
if (result == null) {
console.warn("Unable to find a match");
} else {
console.log("Found match: %k", result);
}

javascript expressions names [closed]

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I am a complete beginner to javascript and I have several things I need to correct on a form in order for it to work. I have to make sure it doesn't reject any valid names (names with accents, hyphens, names with spaces between them). At the moment my regular expression is -
var alphabetic = /^[a-zA-Z]+$/;
if ((alphabetic.test(fname)== false) || (alphabetic.test(lname)== false))
{
alertmsg = alertmsg + "Name should be in alphabets:" + "\n";
}
If someone could point me in the right direction, I would be very grateful

Try this regex :
var alphabetic = /^[a-zàâçéèêëîïôûùüÿñ-\s]+$/i

As Philippe recommended, if you would like to accept languages/alphabets other than English, I would consider more carefully which letters to include. [a-zA-Z] does not seem to recognize letters other than strictly 'A' to 'Z' in my testing.

identifying [] in an expression using regular expression [closed]

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How to get this string to give me the exact matching result
session[username_or_email]
the expression
<html>
<body>
<script type="text/javascript">
var str="session[username_or_email]";
var patt1=/ID|un|name|login_username|userid|username|user|Email|uname|usr|log|email|mail|nick|CUST|account|wpName1|textbox|pw|session[username_or_email]/i;
document.write(str.match(patt1));
</script>
</body>
</html>
the result now is username
Thankyou.

Just escape the [ with \[ so it isn't treated as a special regex control character, but just a normal character to search for.
var str="session[username_or_email]";
var patt1=/ID|un|name|login_username|userid|username|user|Email|uname|usr|log|email|mail|nick|CUST|account|wpName1|textbox|pw|session\[username_or_email]/i;
document.write(str.match(patt1));
You can see it work here: http://jsfiddle.net/jfriend00/CuWKV/
You don't actually have to escape the ] because it is only an expected regex control character when a [ has come before it though it does not harm to escape it also.

Please note the escaped \[ - I also moved the wanted string to the beginning since it contains username too - this may or may not be what you need, but you should consider it.
var str="session[username_or_email]";
var patt1=/session\[username_or_email\]|ID|un|name|login_username|userid|username|user|Email|uname|usr|log|email|mail|nick|CUST|account|wpName1|textbox|pw/i;
document.write(str.match(patt1));