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Capturing after the nth occurrence of a string using regex

My test string:
/custom-heads/food-drinks/51374-easter-bunny-cake
I am trying to capture the number in the string. The constants in that string are the the number is always preceded by 3 /'s and followed by a -.
I am a regex noob and am struggling with this. I cobbled together (\/)(.*?)(-) and then figured I could get the last one programmatically, but I would really like to understand regex better and would love if someone could show me the regex to get the last occurrence of numbers between / and -.

Don't use regexes if possible, i reccomend you to read - https://blog.codinghorror.com/regular-expressions-now-you-have-two-problems/ blog post
To your question, its easier, faster, more bullet proof to get it using splits
const articleName = "/custom-heads/food-drinks/51374-easter-bunny-cake".split("/")[3]
// '51374-easter-bunny-cake'
const articleId = articleName.split("-")[0]
// '51374'
hope it helps

You may use this regex with a capture group:
^(?:[^\/]*\/){3}([^-]+)
Or in modern browsers you can use lookbehind assertion:
/(?<=^(?:[^\/]*\/){3})[^-]+/
RegEx Demo 1
RegEx Demo 2
RegEx Code:
^: Start
(?:[^\/]*\/){3}: Match 0 or more non-/ characters followed by a /. Repeat this group 3 times
([^-]+): Match 1+ of non-hyphen characters
Code:
const s = `/custom-heads/food-drinks/51374-easter-bunny-cake`;
const re = /^(?:[^\/]*\/){3}([^-]+)/;
console.log (s.match(re)[1]);

Use
const str = `/custom-heads/food-drinks/51374-easter-bunny-cake`
const p = /(?:\/[^\/]*){2}\/(\d+)-/
console.log(str.match(p)?.[1])
See regex proof.
EXPLANATION
Non-capturing group (?:\/[^\/]*){2}
{2} matches the previous token exactly 2 times
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
Match a single character not present in the list below [^\/]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
1st Capturing Group (\d+)
\d matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)

how does javascript regex lazy match work?

For this string
abc.com/file/some.png?v=123
how do I match .png? I use
/\..*?\?/
but it is matching .com/file/some.png?, so why is the lazy match rule not working here?

There are lots of variants to this answer. I will propose matching the first file suffix after the last / character.
That can be done with this regex
/(?!.*\/)\.\w+\?/
Explaination
(?!.*/)\.\w+\?
Options: Case insensitive; Dot doesn’t match line breaks; ^$ match at line breaks
Assert that it is impossible to match the regex below starting at this position (negative lookahead) (?!.*/)
Match any single character that is NOT a line break character (line feed, carriage return, line separator, paragraph separator) .*
Between zero and unlimited times, as many times as possible, giving back as needed (greedy) *
Match the character “/” literally /
Match the character “.” literally \.
Match a single character that is a “word character” (ASCII letter, digit, or underscore only) \w+
Between one and unlimited times, as many times as possible, giving back as needed (greedy) +
Match the question mark character \?
\1
Insert a backslash \
Insert the character “1” literally 1
Created with RegexBuddy

Issue with javascript regex not matching less than 3 characters

I have the following javascript regex:
/^[^\s][a-z0-9 ]+[^\s]$/i
I need to allow any alphanumeric character as well as spaces inside the string but not at the beginning nor at the end.
Oddly enough, the above regex will not accept less than 3 characters, e.g. aa will not match but aaa will.
I am not sure why. Can anyone please help ?

You have: [^\s] (requires matching at least one non-whitespace character), [a-z0-9 ]+ (requires matching at least one alphanumeric or space character), and [^\s] again (requires matching at least one non-whitespace character). So, in total, you need at least 3 characters in the string.
Use word boundaries at the beginning and end instead:
/^\b[a-z0-9 ]+\b$/i
https://regex101.com/r/2GhH3N/1

Try the following regex:
^(?! )[a-z0-9 ]*[a-z0-9]$
Details:
^(?! ) - Start of the string and no space after it (so here we exclude the
initial space).
[a-z0-9 ]* - A sequence of letters, digits and spaces, possibly empty
(the content before the last letter(see below).
[a-z0-9]$ - The last letter and the end of string (so here we exclude the
terminal space).

You should re-write the expression as
/^[a-z0-9]+(?:\s+[a-z0-9]+)*$/i
See the regex demo.
NOTE: If only one whitespace is allowed between the alphanumeric chars use
/^[a-z0-9]+(?:\s[a-z0-9]+)*$/i
^^
Details
^ - start of string
[a-z0-9]+ - 1+ letters/digits
(?:\s+[a-z0-9]+)* - 0 or more repetitions of 1+ whitespaces (\s+) and 1+ digit/letters
$ - end of string.
See the regex graph:

Capture between pattern of digits

I'm stuck trying to capture a structure like this:
1:1 wefeff qwefejä qwefjk
dfjdf 10:2 jdskjdksdjö
12:1 qwe qwe: qwertyå
I would want to match everything between the digits, followed by a colon, followed by another set of digits. So the expected output would be:
match 1 = 1:1 wefeff qwefejä qwefjk dfjdf
match 2 = 10:2 jdskjdksdjö
match 3 = 12:1 qwe qwe: qwertyå
Here's what I have tried:
\d+\:\d+.+
But that fails if there are word characters spanning two lines.
I'm using a javascript based regex engine.

You may use a regex based on a tempered greedy token:
/\d+:\d+(?:(?!\d+:\d)[\s\S])*/g
The \d+:\d+ part will match one or more digits, a colon, one or more digits and (?:(?!\d+:\d)[\s\S])* will match any char, zero or more occurrences, that do not start a sequence of one or more digits followed with a colon and a digit. See this regex demo.
As the tempered greedy token is a resource consuming construct, you can unroll it into a more efficient pattern like
/\d+:\d+\D*(?:\d(?!\d*:\d)\D*)*/g
See another regex demo.
Now, the () is turned into a pattern that matches strings linearly:
\D* - 0+ non-digit symbols
(?: - start of a non-capturing group matching zero or more sequences of:
\d - a digit that is...
(?!\d*:\d) - not followed with 0+ digits, : and a digit
\D* - 0+ non-digit symbols
)* - end of the non-capturing group.

you can use or not the ñ-Ñ, but you should be ok this way
\d+?:\d+? [a-zñA-ZÑ ]*
Edited:
If you want to include the break lines, you can add the \n or \r to the set,
\d+?:\d+? [a-zñA-ZÑ\n ]*
\d+?:\d+? [a-zñA-ZÑ\r ]*
Give it a try ! also tested in https://regex101.com/
for more chars:
^[a-zA-Z0-9!##\$%\^\&*)(+=._-]+$

Match Specific File Path Regex

so far I have this regex $fileregex = /([a-z]:\\\\([^\\\\^\\.])*)|(\/[^\/.])/i; but I am very confused on what to do next.
I want to match strings in this format
c:\\something\\else\\something
c:\\something\\else\\something.whatever
/etc/whatever/something/here
/etc/here.txt
/
c:\\
But I don't want to match, for example
c:\oneslash\text.txt
\etc\hi
I am really stuck on my regex especially on repeating the optional path, as one could just request the root.Can anyone help me out with the regex?

This one should work:
preg_match_all('%[A-Za-z]:\\\\\\\\(.*?\\\\\\\\)*.*|/(.*?/)*.*%m', $input, $regs, PREG_PATTERN_ORDER);
for ($i = 0; $i < count($regs[0]); $i++) {
# Matched text = $regs[0][$i];
}
Result:
Description of the Regex:
Match either the regular expression below (attempting the next alternative only if this one fails)
[A-Za-z] Match a single character present in the list below
A character in the range between “A” and “Z”
A character in the range between “a” and “z”
: Match the character “:” literally
\\\\ Match the character “\” literally
\\\\ Match the character “\” literally
( Match the regular expression below and capture its match into backreference number 1
. Match any single character that is not a line break character
*? Between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\\\\ Match the character “\” literally
\\\\ Match the character “\” literally
)* Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
. Match any single character that is not a line break character
* Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
| Or match regular expression number 2 below (the entire match attempt fails if this one fails to match)
/ Match the character “/” literally
( Match the regular expression below and capture its match into backreference number 2
. Match any single character that is not a line break character
*? Between zero and unlimited times, as few times as possible, expanding as needed (lazy)
/ Match the character “/” literally
)* Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
. Match any single character that is not a line break character
* Between zero and unlimited times, as many times as possible, giving back as needed (greedy)

Have you try this:
/^([a-zA-Z]\:|\\\\[^\/\\:*?"<>|]+\\[^\/\\:*?"<>|]+)(\\[^\/\\:*?"<>|]+)+(\.[^\/\\:*?"<>|]+)$/
This regular expression will match any valid file path. It checks local drives and network path. The file extension is required.
Further references : The regular expression library