Regex to Match Alphanumeric and Spaces

Regex for allowing alphanumeric,-,_ and space

Try this regex:

/^[a-z\d\-_\s]+$/i

Regex: Match alphanumeric and spaces with exception of leading spaces

You can use this regex

^[^a-z0-9]+|[^a-z0-9 ]+|\s{2,}|[^a-z0-9]+$

^[^a-z0-9]+ - Matches anything else than alphanumeric at start of string
[^a-z0-9 ]+ - Matches anything else than alphanumeric and space
\s{2,} - Matches two or more consecutive space characters
[^a-z0-9 ]+$ - Matches anything else than alphanumeric at end of string

Demo

Regular Expression for alphanumeric and space

Because Prajeesh only wants to match spaces, \s will not suffice as it matches all whitespace characters including line breaks and tabs.

A character set that should universally work across all RegEx parsers is:

[a-zA-Z0-9 ]

Further control depends on your needs. Word boundaries, multi-line support, etc... I would recommend visiting Regex Library which also has some links to various tutorials on how Regular Expression Parsing works.

Regex to match alphanumeric and spaces

just a FYI

string clean = Regex.Replace(q, @"[^a-zA-Z0-9\s]", string.Empty);

would actually be better like

string clean = Regex.Replace(q, @"[^\w\s]", string.Empty);

Regex for allowing alphanumeric and space

Just add a space; ^[a-zA-Z0-9 ]*$

Increase your google-fu: https://www.google.com/search?q=regex+allow+spaces

I'm not sure how or what you tried googling but there's heaps of results to your question... Including this StackOverflow Post where your question has been answered in great detail.

Regex to allow alphanumeric, spaces, some special characters

You seem to need to avoid matching strings that only consist of digits and make sure the strings start with an alphanumeric. I assume you also need to be able to match empty strings (the original regex matches empty strings).

That is why I suggest

^(?!\d+$)(?:[a-zA-Z0-9][a-zA-Z0-9 @&$]*)?$

See the regex demo

Details

^ - start of string
(?!\d+$) - the negative lookahead that fails the match if a string is numeric only
(?:[a-zA-Z0-9][a-zA-Z0-9 @&$]*)? - an optional sequence of:
- [a-zA-Z0-9] - a digit or a letter
- [a-zA-Z0-9 @&$]* - 0+ digits, letters, spaces, @, & or $ chars
$ - end of string.

Regular expression to allow alphanumeric with space and disallow certain special characters(<>&;)

The following regex matches everything except <>&;:

const regex = /^[^<>&;]*$/
console.log(regex.test('Abc@123'))console.log(regex.test('Abc<123'))

Regex to validate an alphanumeric string with optional space

(?=^.{6,7}$)^(([0-9] ?){4}( ?[a-zA-Z]){2})$

will match

1111 ZZ
111 1ZZ
1 111ZZ
1111ZZ
1111 ZZ

but not

9999 1A
11111 Z
1111111
11 11 ZZ

https://regex101.com/r/lByOx6/1

EDIT: explanation

The "Positive Lookahead" part:

(?=^.{6,7}$) this only matches if the string meets the requirements, BUT it does not 'consume' the characters.
- . is any character
- {6,7} is about repetitions

so (?=^.{6,7}$) is matched if the string has 6 or 7 characters, no matter what

Then the following part already 'consumes' the string to say that I want at the start 4 repetitions of numbers and optionally space, and at the end 2 repetitions of letters and optionally space. The second part would accept strings such as 1 1 1 1 Z Z but as those are more than 7 characters, the first part wouldn't let the string match.

Regex for alphanumeric and single spaces

You can use this regex which uses a negative look ahead to discard the string that starts with a digit, and otherwise captures the text, which can be only separated with one space only,

^(?!\d)[a-zA-Z\d]+(?: [a-zA-Z\d]+)*$

Explanation of regex:

^ - Start of string
(?!\d) - Rejects the match if the string starts with a number
[a-zA-Z\d]+ - Captures one or more alphanumeric characters
(?: [a-zA-Z\d]+)* - Further captures zero or more single space separated alphanumeric characters
$ - End of string

Demo

Allow only alphanumeric characters or empty spaces with Regex

It is because part after | doesn't have assertions of anchors ^ and $.

You can use:

^$|^[a-zA-Z0-9]+$

Or:

^[a-zA-Z0-9]*$

Or:

^(?:[a-zA-Z0-9]+)?$

They will all be doing same thing.