Develop Reference

how to found 2 matches in regular expression

I need a regular expression for :
<<12.txt>> <<45.txt>
I have created a regular expression :
<<.+.txt>>
But this found one match in whole string but here is 2 matches:
<<12.txt>>
<<45.txt>>
if anyone have solution for this problem please help me out there

Part of the issue is that the string you've specified wouldn't match because the second > is missing in <<45.txt>.
Also, you're using the . (dot) selector, and also trying to find a period. It works, but now how you think it is.
Here's the regex you want:
var regex = /<<\d+\.txt>>/g
\d matches only numbers
\. matches an actual period
/g means global, so it won't stop at the first match
Practice Regular Expressions
https://regexr.com/43bs4
Demo
var string = "<<12.txt>> <<45.txt>>";
var regex = /<<\d+\.txt>>/g;
var matches = string.match(regex);
console.log(matches);
P.S., if you actually want to match with 1 > or 2 >>, you can with:
var regex = /<<\d+\.txt>>?/g
? optionally matches the character right before it

/<<.+.txt>>/gm
g is for global (will search through entire source)
m is for multi line search support

Javascript: Remove trailing chars from string if they are non-numeric

I am passing codes to an API. These codes are alphanumeric, like this one: M84.534D
I just found out that the API does not use the trailing letters. In other words, the API is expecting M84.534, no letter D at the end.
The problem I am having is that the format is not the same for the codes.
I may have M84.534DAC, or M84.534.
What I need to accomplish before sending the code is to remove any non-numeric characters from the end of the code, so in the examples:
M84.534D -> I need to pass M84.534
M84.534DAC -> I also need to pass M84.534
Is there any function or regex that will do that?
Thank you in advance to all.

You can use the regex below. It will remove anything from the end of the string that is not a number
let code = 'M84.534DAC'
console.log(code.replace(/[^0-9]+?$/, ""));
[^0-9] matches anything that is not a numer
+? Will match between 1 and unlimited times
$ Will match the end of the string
So linked together, it will match any non numbers at the end of the string, and replace them with nothing.

You could use the following expression:
\D*$
As in:
var somestring = "M84.534D".replace(/\D*$/, '');
console.log(somestring);
Explanation:
\D stands for not \d, the star * means zero or more times (greedily) and the $ anchors the expression to the end of the string.

Given your limited data sample, this simple regular expression does the trick. You just replace the match with an empty string.
I've used document.write just so we can see the results. You use this whatever way you want.
var testData = [
'M84.534D',
'M84.534DAC'
]
regex = /\D+$/
testData.forEach((item) => {
var cleanValue = item.replace(regex, '')
document.write(cleanValue + '<br>')
})
RegEx breakdown:
\D = Anything that's not a digit
+ = One or more occurrences
$ = End of line/input

Use Regex to Replace the Starting substring

The following statement needs to be replaced, as in the following code
page_url.replace("www.", "");
This needs to be done with the use of regex. Please let me know the method of achieving the same.
Logically, it needs to replace the "www." with "" if and only if the string starts with www..

"www.test.com".replace(/^www\./, "") // returns "test.com"
or in long form:
var regex = /^www\./;
var stringToMatch = "www.test.com";
var result = stringToMatch.replace(regex, ""); // "test.com"
where the /^www\./ defines the regex object. This is made up of:
/ start of the regex
^ start of string, means this match must appear at the begining
www match the characters www
\. match the character .. We must escape it with \ because in regex . means match anything
/ end of the regex definition
If you wan't to play about with the regex to see how it works, try it in this web regex tester: https://regex101.com/r/7ErXz8/2

Regular expression with asterisk quantifier

This documentation states this about the asterisk quantifier:
Matches the preceding character 0 or more times.
It works in something like this:
var regex = /<[A-Za-z][A-Za-z0-9]*>/;
var str = "<html>";
console.log(str.match(regex));
The result of the above is : <html>
But when tried on the following code to get all the "r"s in the string below, it only returns the first "r". Why is this?
var regex = /r*/;
var str = "rodriguez";
console.log(str.match(regex));
Why, in the first example does it cause "the preceding" character/token to be repeated "0 or more times" but not in the second example?

var regex = /r*/;
var str = "rodriguez";
The regex engine will first try to match r in rodriguez from left to right and since there is a match, it consumes this match.
The regex engine then tries to match another r, but the next character is o, so it stops there.
Without the global flag g (used as so var regex = /r*/g;), the regex engine will stop looking for more matches once the regex is satisfied.
Try using:
var regex = /a*/;
var str = "cabbage";
The match will be an empty string, despite having as in the string! This is because at first, the regex engine tries to find a in cabbage from left to right, but the first character is c. Since this doesn't match, the regex tries to match 0 times. The regex is thus satisfied and the matching ends here.
It might be worth pointing out that * alone is greedy, which means it will first try to match as many as possible (the 'or more' part from the description) before trying to match 0 times.
To get all r from rodriguez, you will need the global flag as mentioned earlier:
var regex = /r*/g;
var str = "rodriguez";
You'll get all the r, plus all the empty strings inside, since * also matches 'nothing'.

Use global switch to match 1 or more r anywhere in the string:
var regex = /r+/g;
In your other regex:
var regex = /<[A-Za-z][A-Za-z0-9]*>/;
You're matching literal < followed by a letter followed by 0 or more letter or digits and it will perfectly match <html>
But if you have input as <foo>:<bar>:<abc> then it will just match <foo> not other segments. To match all segments you need to use /<[A-Za-z][A-Za-z0-9]*>/g with global switch.

how to regex a string between two tokens in Javascript?

Asked many times, but I can't get it to work...
I have strings like:
"text!../tmp/widgets/tmp_widget_header.html"
and am trying like this to extract widget_header:
var temps[i] = "text!../tmp/widgets/tmp_widget_header.html";
var thisString = temps[i].regexp(/.*tmp_$.*\.*/) )
but that does not work.
Can someone tell me what I'm doing wrong here?
Thanks!

This prints widget_header:
var s = "text!../tmp/widgets/tmp_widget_header.html";
var matches = s.match(/tmp_(.*?)\.html/);
console.log(matches[1]);

var s = "text!../tmp/widgets/tmp_widget_header.html",
re = /\/tmp_([^.]+)\./;
var match = re.exec(s);
if (match)
alert(match[1]);
This will match:
a / character
the characters tmp_
one or more of any character that is not the . character. These are captured.
a . character
If a match was found, it will be at index 1 of the resulting Array.

In your code:
var temps[i] = "text!../tmp/widgets/tmp_widget_header.html";
var thisString = temps[i].regexp(/.*tmp_$.*\.*/) )
You are saying:
"Match any string that starts with any number of any characters, followed by "tmp_", followed by the end of input, followed by any number of periods."
.* : Any number of any character (except newline)
tmp_ : Literally "tmp_"
$ : End of input/newline - this will never be true in this position
\. : " . ", a period
\.* : Any number of periods
Plus when using the regex() function you need to pass a string, using string notation like var re = new RegExp("ab+c") or var re = new RegExp('ab+c') not in regex notation using slash. You also have either an extra, or missing parenthesis, and no characters are actually being captured.
What you want to do is:
"Find a string that preceded by the begining of input, followed by one or more of any character, followed by "tmp_"; followed by a single period, followed by one or more of any character, followed by the end of input;t that contains one or more of any character. Capture that string."
So:
var string = "text!../tmp/widgets/tmp_widget_header.html";
var re = /^.+tmp_(.+)\..+$/; //I use the simpler slash notation
var out = re.exec(string); //execute the regex
console.log(out[1]); //Note that out is an array, the first (here only) catpture sting is at index 1
This regex /^.+tmp_(.+)\..+$/ means:
^ : Match beginning of input/line
.+ : One or more of any character (except newline), "+" is one or more
tmp_ : Constant "tmp_"
\. : A single period
.+ : As above
$ : End of input/line
You could also use this as RegEx('^.+tmp_(.+)\..+$'); not that when we use RegEx(); we do not have the slash marks, instead we use quote marks (single or double will work), to pass it as a string.
Now this would also match var string = "Q%$#^%$^%$^%$^43etmp_ebeb.45t4t#$^g" and out == 'ebeb'. So depending on the specific use you may wish to replace any " . " used to signify any character (except newline) with bracketed "[ ]" character lists, as this may filter out unwanted results. You milage may vary.
For more information visit: https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Regular_Expressions