Develop Reference

Regex to validate optional parts of a bigger format

The general format is:
YYYY/MM/DD/INFO
Only the separators / are mandatory.
Each part is optional.
YYYY - exactly 4 numbers.
MM - exactly 2 numbers.
DD - exactly 2 numbers.
INFO - any sequence of letters, spaces or hyphens.
So these are valid strings:
2020/06/25/XYZConf
2020///XYZConf
2020//25/XYZConf
2020/06//XYZConf
//25/XYZConf
///
I'm really struggling to come up with a regex that validates optional parts while maintaining the integrity of the string as a whole.
How would you write this regular expression?
PS: This needs to be a regular expression as it will be part of a third-party lexer that doesn't accept anything else.
Relevant posts:
Regex to validate comma-separated numbers with optional fractional parts
Regex for dates that matches for every stage of valid date entry

You could try something like:
^(?:\d{4})?\/(?:(?:\d\d)?\/){2}(?:[A-Za-z\s-]+)?$
See the Online Demo
I believe that you are looking for optional (non)capturing groups. The pattern above matches:
^ - Start string ancor.
(?: - Open 1st non-capturing group.
\d{4} - Match 4 digits.
)? - Close 1st non-capturing group and make it optional.
\/ - Match a forward slash.
(?: - Open 2nd non-capturing group.
(?: - Open 3rd non-capturing group.
\d\d - Match two digits.
)? - Close 3rd non-capturing group and make it optional.
\/ - Match a forward slash.
){2} - Close 2nd non-capturing group and make it match twice.
(?: - Open 4th non-capturing group.
[A-Za-z\s-]+ - Match upper- and lowercase letters, a space and hyphen at least one time (in any sequence as per your OP).
)? - Close 4th non-capturing group and make it optional.
$ - End string ancor.

Making dot optional with limited characters

I am trying the following Regex and It is failing
/^\d{1,18}[.]?$/
I want digit 1-18 but a optional dot(.) anywhere. I tried the following too
/^[1-9]{1,18}[.]?$/
It counts . as a character as well i.e 12345678901234567.
How can I achieve 18 digits and an optional . anywhere in regex

You may use this regex with a lookahead to block 2 dots:
^(?!(?:\d*\.){2})[.\d]{1,18}$
RegEx Demo
RegEx Details:
^: Start
(?!(?:\d*\.){2}): Negative lookahead to disallow 2 dots
[.\d]{1,18}: Match dot or digit for length range between 1 to 18
$: End

You may use
^(?=(?:\.?\d){1,18}\.?$)\d*\.?\d*$
See the regex demo
Details
^ - start of string
(?=(?:\.?\d){1,18}\.?$) - a positive lookahead that requires 1 to 18 occurrences of an optional . and any digit followed with an optional . at the end of string
\d*\.?\d* - 0+ digits, an optional . and again 0+ digits
$ - end of string.

Regex for d h m time string

I have a problem to check my string for correct time input.
It should be possible to write it like this: (number)d (number)h (number)m
Each part can be left out and the numbers don't have a limit (e.g. 2352h).
Some possibilities are the following:
2424d 23h 15m
523h 526m
235d 252m
829d 926h
I use the following RegEx which works really good but has one problem, which is that one letter can be used multiple times (e.g. 52d 23d).
I want that each letter can be used once. Whitespaces are optional.
This is what I have for now:
^((\d{0,9}[dh]\s?){0,1}(\d{0,9}m)?){1,3}\s*$
Can anyone help me?

Try this Regex:
^(?=.*[hmd]$)\d+(?:d\s*)?\d*(?:h\s*)?\d*(?:m\s*)?$
Click for Demo
Explanation
^ - asserts the start of the string
(?=.*[hmd]$) - positive lookahead to make sure that the string ends with either d, h or m
\d+(?:d\s*)? - matches 1+ occurrences of a digit followed by optional d followed by 0+ spaces
\d*(?:h\s*)? - matches 0+ occurrences of a digit followed by optional h followed by 0+ spaces
\d*(?:m\s*)? - matches 0+ occurrences of a digit followed by optional m followed by 0+ spaces
$ - asserts the end of the string
Another alternative: ^(?=.*[hmd]$)(?:\d+d\s*)?(?:\d+h\s*)?(?:\d+m\s*)?$

Generic JavaScript regex validating a positive number with or without commas as thousand separators and an optional fractional part

I have following regex to validate numbers in input
var reg = /^\d+$/;
Now i want to allow ,(commas) and .(period) in number field as following will some one help me writing regex to allow following number format ?
10000000
10,000,000
10000000.00

You may use
/^(?:\d{1,3}(?:,\d{3})+|\d+)(?:\.\d+)?$/
See the regex demo
If you only need to allow 2 digits after the decimal separator, replace (?:\.\d+)? with (?:\.\d{1,2})?.
Details:
^ - start of string
(?:\d{1,3}(?:,\d{3})*|\d+) - 2 alternatives:
\d{1,3}(?:,\d{3})+ - 1 to 3 digits and one or more sequences of a comma and 3 digits
\d+ - 1 or more digits
(?:\.\d+)? - an optional sequence of:
\. - a dot
\d+ - 1 or more digits
$

You could use
^((\d{1,2}(,\d{3})+)|(\d+)(\.\d{2})?)$
see Regex101
or
^((\d{1,2}(,\d{3})+)|(\d+))(\.\d{2})?$
if you want 10,000,000.00 to get matched to.

Regular expression for thousand separator without decimal point

I have regular expression which is working fine for decimal .
In below expression I want to remove decimal match to validate Integer
^-?(\d*)(,\d{1,3})*(?:[,]|([.]\d{0,2}))?$
Valid Match : 1,000 & 111, & -1000,00
Invalid Match : -,1 & 1,,,,

Use
^-?(?:\d{1,3}(?:,\d{3})*|\d+)?,?$
|---1---|
|---2-----|
|-3-|
The ^ asserts the position at the start of the string and $ asserts the position at the string end. -? matches an optional -. Part 1 matches 1 to 3 digits, Part 2 matches 0+ sequences of , followed with 3 digits, part 3 is an alternative to Part 1 & 2 combined and matches just 1+ digits. The whole number pattern is optional since the (?:...|...) is quantified with ? - one or zero occurrences.
The ,? before $ allows an optional comma at the end, delete if not required (you mentioned you need to allow 100, in the comments).
See the regex demo