Develop Reference

How do I replace the last character of the selected regex?

I want this string {Rotation:[45f,90f],lvl:10s} to turn into {Rotation:[45,90],lvl:10}.
I've tried this:
const bar = `{Rotation:[45f,90f],lvl:10s}`
const regex = /(\d)\w+/g
console.log(bar.replace(regex, '$&'.substring(0, -1)))
I've also tried to just select the letter at the end using $ but I can't seem to get it right.

You can use
bar.replace(/(\d+)[a-z]\b/gi, '$1')
See the regex demo.
Here,
(\d+) - captures one or more digits into Group 1
[a-z] - matches any letter
\b - at the word boundary, ie. at the end of the word
gi - all occurrences, case insensitive
The replacement is Group 1 value, $1.
See the JavaScript demo:
const bar = `{Rotation:[45f,90f],lvl:10s}`
const regex = /(\d+)[a-z]\b/gi
console.log(bar.replace(regex, '$1'))

Check this out :
const str = `{Rotation:[45f,90f],lvl:10s}`.split('');
const x = str.splice(str.length - 2, 1)
console.log(str.join(''));

You can use positive lookahead to match the closing brace, but not capture it. Then the single character can be replaced with a blank string.
const bar= '{Rotation:[45f,90f],lvl:10s}'
const regex = /.(?=})/g
console.log(bar.replace(regex, ''))
{Rotation:[45f,90f],lvl:10}

The following regex will match each group of one or more digits followed by f or s.
$1 represents the contents captured by the capture group (\d).
const bar = `{Rotation:[45f,90f],lvl:10s}`
const regex = /(\d+)[fs]/g
console.log(bar.replace(regex, '$1'))

Regex match apostrophe inside, but not around words, inside a character set

I'm counting how many times different words appear in a text using Regular Expressions in JavaScript. My problem is when I have quoted words: 'word' should be counted simply as word (without the quotes, otherwise they'll behave as two different words), while it's should be counted as a whole word.
(?<=\w)(')(?=\w)
This regex can identify apostrophes inside, but not around words. Problem is, I can't use it inside a character set such as [\w]+.
(?<=\w)(')(?=\w)|[\w]+
Will count it's a 'miracle' of nature as 7 words, instead of 5 (it, ', s becoming 3 different words). Also, the third word should be selected simply as miracle, and not as 'miracle'.
To make things even more complicated, I need to capture diacritics too, so I'm using [A-Za-zÀ-ÖØ-öø-ÿ] instead of \w.
How can I accomplish that?

1) You can simply use /[^\s]+/g regex
const str = `it's a 'miracle' of nature`;
const result = str.match(/[^\s]+/g);
console.log(result.length);
console.log(result);
2) If you are calculating total number of words in a string then you can also use split as:
const str = `it's a 'miracle' of nature`;
const result = str.split(/\s+/);
console.log(result.length);
console.log(result);
3) If you want a word without quote at the starting and at the end then you can do as:
const str = `it's a 'miracle' of nature`;
const result = str.match(/[^\s]+/g).map((s) => {
s = s[0] === "'" ? s.slice(1) : s;
s = s[s.length - 1] === "'" ? s.slice(0, -1) : s;
return s;
});
console.log(result.length);
console.log(result);

You might use an alternation with 2 capture groups, and then check for the values of those groups.
(?<!\S)'(\S+)'(?!\S)|(\S+)
(?<!\S)' Negative lookbehind, assert a whitespace boundary to the left and match '
(\S+) Capture group 1, match 1+ non whitespace chars
'(?!\S) Match ' and assert a whitespace boundary to the right
| Or
(\S+) Capture group 2, match 1+ non whitespace chars
See a regex demo.
const regex = /(?<!\S)'(\S+)'(?!\S)|(\S+)/g;
const s = "it's a 'miracle' of nature";
Array.from(s.matchAll(regex), m => {
if (m[1]) console.log(m[1])
if (m[2]) console.log(m[2])
});

Reg Exp for finding hashtag words

I have the following sentence as a test:
This is a test with #shouldshow and to see if there #show
#yes this#shouldnotshow what is going on here
I have figured out most of the Reg Exp I need. Here's what I have so far: /((?<=#)([A-Z]*))/gi
This matches every tag but also matches the shouldnotshow portion. I want to not match words that are prefixed by anything but # (excluding whitespace & \n).
So the only matched words I should get are: shouldshow show yes.
Note: after #show is a newline

You just need to see if the hash is prefixed with whitespace or starts the string
https://regex101.com/r/JDuGvr/1
/(\s|^)#(\w+)/gm
with positive lookbehind as OP used
https://regex101.com/r/06X3ZX/1
/(?<=(\s|^)#)(\w+)/gm;
use [a-zA-Z0-9] if you do not want an underscore
const re1 = /(\s|^)#(\w+)/gm;
const re2 = /(?<=(\s|^)#)(\w+)/gm;
const str = `This is a test with #shouldshow and to see if there #show
#yes this#shouldnotshow what is going on here`;
const res1 = [...str.matchAll(re1)].map(match => match[2]); // here the match is the third item
console.log(res1)
const res2 = [...str.matchAll(re2)].map(match => match[0]); // match is the first item
console.log(res2)

Another option could be using your pattern asserting a # on the left that does not have a non whitespace char before it using (?<!\S)# and get the match only without capture groups.
Match at least 1+ times a char A-Z to prevent matching an empty string.
(?<=(?<!\S)#)[A-Z]+
Regex demo
const regex = /(?<=(?<!\S)#)[A-Z]+/gi;
const str = `This is a test with #shouldshow and to see if there #show
#yes this#shouldnotshow what is going on her`;
console.log(str.match(regex));

Setting the end of the match

I have the following string:
[TITLE|prefix=a] [STORENAME|prefix=b|suffix=c] [DYNAMIC|limit=10|random=0|reverse=0]
And I would like to get the value of the prefix of TITLE, which is a.
I have tried it with (?<=TITLE|)(?<=prefix=).*?(?=]|\|) and that seems to work but that gives me also the prefix of STORENAME (b). So if [TITLE|prefix=a] will be missing in the string, I'll have the wrong value.
So I need to set the end of the match with ] that belongs to [TITLE. Please notice that this string is dynamic. So it could be [TITLE|suffix=x|prefix=y] as well.
const regex = "[TITLE|prefix=a] [STORENAME|prefix=b|suffix=c] [DYNAMIC|limit=10|random=0|reverse=0]".match(/(?<=TITLE|)(?<=prefix=).*?(?=]|\|)/);
console.log(regex);

You can use
(?<=TITLE(?:\|suffix=[^\]|]+)?\|prefix=)[^\]|]+
See the regex demo. Details:
(?<=TITLE(?:\|suffix=[^\]|]+)?\|prefix=) - a location in string immediately preceded with TITLE|prefix| or TITLE|suffix=...|prefix|
[^\]|]+ - one or more chars other than ] and |.
See JavaScript demo:
const texts = ['[TITLE|prefix=a] [STORENAME|prefix=b|suffix=c] [DYNAMIC|limit=10|random=0|reverse=0]', '[TITLE|suffix=s|prefix=a]'];
for (let s of texts) {
console.log(s, '=>', s.match(/(?<=TITLE(?:\|suffix=[^\]|]+)?\|prefix=)[^\]|]+/)[0]);
}

You could also use a capturing group
\[TITLE\|(?:[^|=\]]*=[^|=\]]*\|)*prefix=([^|=\]]*)[^\]]*]
Explanation
\[TITLE\| Match [TITLE|
(?:\w+=\w+\|)* Repeat 0+ occurrences wordchars = wordchars and |
prefix= Match literally
(\w+) Capture group 1, match 1+ word chars
[^\]]* Match any char except ]
] Match the closing ]
Regex demo
const regex = /\[TITLE\|(?:\w+=\w+\|)*prefix=(\w+)[^\]]*\]/g;
const str = `[TITLE|prefix=a] [STORENAME|prefix=b|suffix=c] [DYNAMIC|limit=10|random=0|reverse=0]
[TITLE|suffix=x|prefix=y]`;
let m;
while ((m = regex.exec(str)) !== null) {
console.log(m[1]);
}
Or with a negated character class instead of \w
\[TITLE\|(?:[^|=\]]*=[^|=\]]*\|)*prefix=([^|=\]]*)[^\]]*]
Regex demo

Making Regex more safe

i'm trying to turn a bunch of regex more safe, what i mean by more safe, i want more accuracy.
So, i'm very new with RegExp, and i want know if i'm doing this right (not the Regex, but turn into more safety).
So, i'm starting now, and this is the first RegExp that i want change, i want push the 01/2011.
Past RegExp:
var text = 'INSCRIÇÃO: 60.537.263/0001-66 COMP: 01/2011 COD REC: 150';
var reg = /COMP.*?(\d\S*)/;
var match = reg.exec(text);
console.log(match[1]);
New RegExp:
var text = 'INSCRIÇÃO: 60.537.263/0001-66 COMP: 01/2011 COD REC: 150';
var reg = /COMP:\s([0-9]{0,2}\/[0-9]{0,4})/;
var match = reg.exec(text);
console.log(match[1]);
Why this? This text is just a part of a huge text, so i need accuraci.
Other question is about turn the Regex optional, so if doesn't match anything, return undefined.
Thanks.

According to your feedback:
i want specifically push the value with two numbers, one / and four numbers
You can use
/\bCOMP:\s*(\d{2}\/\d{4})(?!\d)/g
The \b is a word boundary, thus 5COMP won't be matched.
The \s* will match 0 or more whitespace (if there must be whitespace, use + quantifier instead).
The \d{2} will match exactly 2 digits.
The \d{4} will match 4 digits and no more because of the look-ahead (?!\d). This look-ahead just makes sure there is no digit after the 4 previous digits. You may use \b here as well to ensure matching a word boundary.
arr = [];
var re = /\bCOMP:\s*(\d{2}\/\d{4})(?!\d)/g;
var str = 'COMP:10/9995, COMP: 21/1234, COMP: 21/123434, REGCOMP: 21/1234';
var m;
while ((m = re.exec(str)) !== null) {
arr.push(m[1]);
}
console.log(arr);