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Regular expression to match letters and digits, where some symbols can appear between them

I am trying to create a regular expression to match letters and digits, where .,- and _ can appear between them.
Examples of valid matches: "stephan", "mike03", "s.johnson", "st_steward", "john-johnson", "12345", "324_231351231".
Examples of invalid users: ''--123", ".....", "john_-", "_steve", ".info".
I came up with this expression
[A-Za-z0-9.\-_]+
but it will also match things like stefan_johnson_ which should not be matched because _ can only appear between the letters and the digits. The same holds if we have _ or any of the other already mentioned symbols in the beginning.

You can use
^[A-Za-z0-9]+(?:[._-][A-Za-z0-9]+)*$
See the regex demo. Details:
^ - start of string
[A-Za-z0-9]+ - one or more ASCII letters or digits
(?:[._-][A-Za-z0-9]+)* - zero or more occurrences of
[._-] - a ., _ or -
[A-Za-z0-9]+ - one or more ASCII letters or digits
$ - end of string.
It may turn out you need to support any Unicode letters or digits, then use
^[\p{L}\p{N}]+(?:[._-][\p{L}\p{N}]+)*$
where \p{L} matches any Unicode (base) letter and \p{N} matches any Unicode digit.
Replace $ with \z if you do not want to allow a trailing \n (line feed) char.
Also, see my "Validating strings with comma-separated values (with no leading/trailing separators)" YT video explaining this kind of validation technique.

How can I match four number fields or a text string using regex

So a sensor I'm interfacing to either outputs 4 multi-digit integers (separated by spaces) or an error string.
Ideally my regex would return a match for either of the above scenarios and reject any
other outputs - e.g. if only 3 numbers are output. I can then check if there are 4 groups (number output) or 1 group (error string output) in the following c#.
The regex I have matches all the time and returns spaces when there are less than 4 numbers so I still need to check everything.
I've tried putting in ?: but the format breaks. Any regex whizzes up to the challenge? Thanks in advance.
([0-9]+\s)([0-9]+\s)([0-9]+\s)([0-9]+)|([a-zA-Z\s_!]+)
So a numeric example would be 11 222 33 4444 or Sensor is in an error state! An incorrect output would be 222 11 3333 as it only has 3 fields
Also - I need to capture the four numbers (but not the spaces) or the error string.

You can capture either the 4 groups with only digits and match the whitespace chars outside of the group.
Or else match 1+ times any of the listed characters in the character class. Note that \s can also match a newline, and as the \s is in the character class the match can also consist of only spaces for example.
To match the whole string, you can add anchors.
^(?:([0-9]+)\s([0-9]+)\s([0-9]+)\s([0-9]+)|[a-zA-Z\s_!]+)$
» Regex demo
Another option to match the error string, is to start matching word characters without digits, optionally repeated by a whitespace char and again word characters without digits.
^(?:([0-9]+)\s([0-9]+)\s([0-9]+)\s([0-9]+)|[^\W\d]+(?:\s+[^\W\d]+)*!?)$
» Regex demo
If there can be a digit in the error message, but you don't want to match only digits or whitespace chars, you can exclude that using a negative lookahead.
^(?:([0-9]+)\s([0-9]+)\s([0-9]+)\s([0-9]+)|(?![\d\s]*$).+)$
» Regex demo

C# Regular Expression for x number of groups of A-Z separated by hyphen

I am trying to match the following pattern.
A minimum of 3 'groups' of alphanumeric characters separated by a hyphen.
Eg: ABC1-AB-B5-ABC1
Each group can be any number of characters long.
I have tried the following:
^(\w*(-)){3,}?$
This gives me what I want to an extent.
ABC1-AB-B5-0001 fails, and ABC1-AB-B5-0001- passes.
I don't want the trailing hyphen to be a requirement.
I can't figure out how to modify the expression.

Your ^(\w*(-)){3,}?$ pattern even allows a string like ----- because the only required pattern here is a hyphen: \w* may match 0 word chars. The - may be both leading and trailing because of that.
You may use
\A\w+(?:-\w+){2,}\z
Details:
\A - start of string
\w+ - 1+ word chars (that is, letters, digits or _ symbols)
(?:-\w+){2,} - 2 or more sequences of:
- - a single hyphen
\w+ - 1 or more word chars
\z - the very end of string.
See the regex demo.
Or, if you do not want to allow _:
\A[^\W_]+(?:-[^\W_]+){2,}\z
or to only allow ASCII letters and digits:
\A[A-Za-z0-9]+(?:-[A-Za-z0-9]+){2,}\z

It can be like this:
^\w+-\w+-\w+(-\w+)*$

^(\w+-){2,}(\w+)-?$
Matches 2+ groups separated by a hyphen, then a single group possibly terminated by a hyphen.

((?:-?\w+){3,})
Matches minimum 3 groups, optionally starting with a hyphen, thus ignoring the trailing hyphen.
Note that the \w word character also select the underscore char _ as well as 0-9 and a-z
link to demo

Password Regular Expression Validation

These are the requirements but I guess it's too complicated for my regular expression skills...
. between 6 and 10 alphanum characters
. allowed A-Z,a-z,0-9,#,$,_
. Must begin with a letter
. Must contain at least one number
. cannot contain two consecutive identical characters
. cannot contain two consecutive identical numbers
I know the basic of regular expression such as
[A-Za-Z] = characters only etc... but when it comes to consecutive character and stuff...

Try this
((?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[\W]).{6,20})
Description of above Regular Expression:
( # Start of group
(?=.*\d) # must contains one digit from 0-9
(?=.*[a-z]) # must contains one lowercase characters
(?=.*[A-Z]) # must contains one uppercase characters
(?=.*[\W]) # must contains at least one special character
. # match anything with previous condition checking
{6,20} # length at least 6 characters and maximum of 20
) # End of group
"/W" will increase the range of characters that can be used for password and pit can be more safe.

string pattern1 = #"^[a-zA-Z]([a-zA-Z])*"; //start and any number of characters
string pattern2 = #"[0-9]+"; //one number or more numbers
string pattern3 = #"[##$%]*"; // special symbol allowed
string pattern4 = #"(.)\1";//consecutive characters
string pattern5 = #"^(.){6,10}$"; //min max

should you want to validate the password you can use groups to do soo ;
(?<a>[a-zA-Z])?(?<b>[0-9])?(?<c>[#%$#/\\\(\)])?
Will give you a match in any of the 3 groups (a,b and c)
uper and lower characters will be in group a
numeric characters will be in group b
and special characters will be in group c
you can use the regex.match.groups("a").count to see if any characters from group a could be found
if you find characters in all 3 groups, the password is strong.

Regex match if a string has length 2 and contains 1 letter and 1 number

Guys I hate Regex and I suck at writing.
I have a string that is space separated and contains several codes that I need to pull out. Each code is marked by beginning with a capital letter and ending with a number. The code is only two digits.
I'm trying to create an array of strings from the initial string and I can't get the regular expression right.
Here is what I have
String[] test = Regex.Split(originalText, "([a-zA-Z0-9]{2})");
I also tried:
String[] test = Regex.Split(originalText, "([A-Z]{1}[0-9]{1})");
I don't have any experience with Regex as I try to avoid writing them whenever possible.
Anyone have any suggestions?
Example input:
AA2410 F7 A4 Y7 B7 A 0715 0836 E0.M80
I need to pull out F7, A4, B7. E0 should be ignored.

You want to collect the results, not split on them, right?
Regex regexObj = new Regex(#"\b[A-Z][0-9]\b");
allMatchResults = regexObj.Matches(subjectString);
should do this. The \bs are word boundaries, making sure that only entire strings (like A1) are extracted, not substrings (like the A1 in TWA101).
If you also need to exclude "words" with non-word characters in them (like E0.M80 in your comment), you need to define your own word boundary, for example:
Regex regexObj = new Regex(#"(?<=^|\s)[A-Z][0-9](?=\s|$)");
Now A1 only matches when surrounded by whitespace (or start/end-of-string positions).
Explanation:
(?<= # Assert that we can match the following before the current position:
^ # Start of string
| # or
\s # whitespace.
)
[A-Z] # Match an uppercase ASCII letter
[0-9] # Match an ASCII digit
(?= # Assert that we can match the following after the current position:
\s # Whitespace
| # or
$ # end of string.
)
If you also need to find non-ASCII letters/digits, you can use
\p{Lu}\p{N}
instead of [A-Z][0-9]. This finds all uppercase Unicode letters and Unicode digits (like Ä٣), but I guess that's not really what you're after, is it?

Do you mean that each code looks like "A00"?
Then this is the regex:
"[A-Z][0-9][0-9]"
Very simple... By the way, there's no point writing {1} in a regex. [0-9]{1} means "match exactly one digit, which is exactly like writing [0-9].
Don't give up, simple regexes make perfect sense.

This should be ok:
String[] all_codes = Regex.Split(originalText, #"\b[A-Z]\d\b");
It gives you an array with all code starting with a capital letter followed by a digit, separated by an kind of word boundary (site space etc.)