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Simple phone number regex to match numbers, spaces, etc

I'm trying to modify a fairly basic regex pattern in C# that tests for phone numbers.
The patterns is -
[0-9]+(\.[0-9][0-9]?)?
I have two questions -
1) The existing expression does work (although it is fairly restrictive) but I can't quite understand how it works. Regexps for similar issues seem to look more like this one -
/^[0-9()]+$/
2) How could I extend this pattern to allow brackets, periods and a single space to separate numbers. I tried a few variations to include -
[0-9().+\s?](\.[0-9][0-9]?)?
Although i can't seem to create a valid pattern.
Any help would be much appreciated.
Thanks,

[0-9]+(\.[0-9][0-9]?)?
First of all, I recommend checking out either regexr.com or regex101.com, so you yourself get an understanding of how regex works. Both websites will give you a step-by-step explanation of what each symbol in the regex does.
Now, one of the main things you have to understand is that regex has special characters. This includes, among others, the following: []().-+*?\^$. So, if you want your regex to match a literal ., for example, you would have to escape it, since it's a special character. To do so, either use \. or [.]. Backslashes serve to escape other characters, while [] means "match any one of the characters in this set". Some special characters don't have a special meaning inside these brackets and don't require escaping.
Therefore, the regex above will match any combination of digits of length 1 or more, followed by an optional suffix (foobar)?, which has to be a dot, followed by one or two digits. In fact, this regex seems more like it's supposed to match decimal numbers with up to two digits behind the dot - not phone numbers.
/^[0-9()]+$/
What this does is pretty simple - match any combination of digits or round brackets that has the length 1 or greater.
[0-9().+\s?](\.[0-9][0-9]?)?
What you're matching here is:
one of: a digit, round bracket, dot, plus sign, whitespace or question mark; but exactly once only!
optionally followed by a dot and one or two digits
A suitable regex for your purpose could be:
(\+\d{2})?((\(0\)\d{2,3})|\d{2,3})?\d+
Enter this in one of the websites mentioned above to understand how it works. I modified it a little to also allow, for example +49 123 4567890.
Also, for simplicity, I didn't include spaces - so when using this regex, you have to remove all the spaces in your input first. In C#, that should be possible with yourString.Replace(" ", ""); (simply replacing all spaces with nothing = deleting spaces)

The + after the character set is a quantifier (meaning the preceeding character, character set or group is repeated) at least one, and unlimited number of times and it's greedy (matched the most possible).
Then [0-9().+\s]+ will match any character in set one or more times.

Matching a number preceeded by a know string, followed by an unknown number of characters

[SOME_WORDS:200:1000]
Trying to match just the last 1000 part. Both numbers are variable and can contain an unknown number of characters (although they are expected to contain digits, I cannot rule out that they may also contain other characters). The SOME_WORDS part is known and does not change.
So I begin by doing a positive lookbehind for [SOME_WORDS: followed by a positive lookahead for the trailing ]
That gives us the pattern (?<=\[SOME_WORDS:).*(?=])
And captures the part 200:1000
Now because I don't know how many characters are after SOME_WORDS:, but I know that it ends with another : I use .*: to indicate any character any amount of time followed by :
That gives us the pattern (?<=\[SOME_WORDS:.*:).*(?=])
However at this point the pattern no longer matches anything and this is where I become confused. What am I doing wrong here?
If I assume that the first number will always be 3 characters long I can replace .* with ... to get the pattern (?<=\[SOME_WORDS:...:).*(?=]) and this correctly captures just the 1000 part. However I don't understand why replacing ... with .* makes the pattern not capture anything.
EDIT:
It seems like the online tool I was using to test the regex pattern wasn't working correctly. The pattern (?<=\[SOME_WORDS:.*:).*(?=]) matches the 1000 with no issues when actually done in .net

You usually cannot use a + or a * in a lookbehind, only in a lookahead.
If c# does allow these than you could use a .*? instead of a .* as the .* will eat the second :
Try this:
(?<=\[SOME_WORDS:)(?=\d+:(\d+)])
The match wil be in the first capture group
Quote from http://www.regular-expressions.info/lookaround.html
The bad news is that most regex flavors do not allow you to use just any regex inside a lookbehind, because they cannot apply a regular expression backwards. The regular expression engine needs to be able to figure out how many characters to step back before checking the lookbehind. When evaluating the lookbehind, the regex engine determines the length of the regex inside the lookbehind, steps back that many characters in the subject string, and then applies the regex inside the lookbehind from left to right just as it would with a normal regex.

As Robert Smit mentions this is due to the * being a greedy operator. Greedy operators consume as many characters as they possibly can when they are matched first. They only give up characters if the match fails. If you make the greedy operator lazy(*?), then matching consumes as little number of characters as possible for the match to succeed, so the : is not consumed by *. You can also use [^:]* which is match any character other than :.

Regex for a repeating pattern in C#

I am trying to create a regex to validate a string. The string could be of the following formats (to give an idea of what I am trying to do here):
145/1/3 or
748/57676/6765/454/345 or
45/234 45/235 45/236
So basically the string can contain numbers, spaces and forward slashes and the string can end with a number only. I am new at regex and have gone through many of the questions on the website. But please you have to admit that this is really confusing and difficult to master. And if someone could refer an author or any weblink that can teach regex, that would be really helpful. Thanks in advance mates!

I came up with this
^[0-9]( |[0-9]|\/)*[0-9]$
And used this to test it.
You can see it matches anything that begins (^) with a number, has zero or more (*) of either a space, a number or (|) a forward-slash (/) and ends ($) with a number.
Now that I am aware that the space and / cannot go together and multiple spaces and/or slashes are also not allowed, this RegEx is a better fit for you.
^[0-9]+([ \/][0-9]+)*$

This should work: ^[/\d\s]*\d$.
It is looking for the beginning of the string ^ , then 0 or more digits, spaces [/\d\s]* followed by a digit \d then the end of the string $.

You should use following regular expression:
(\d+(/\d+)*\s*)+
This mean: some digits (\d+) followed by optional repeating pattern of some digits and \ ((/\d+)*) followed by an optional number of whitespaces (\s*), all repeated at least once.

Try this:
^\d(\d|\s|\/)*\d$
\d = digit character (you can also use [0-9]).
\s = space character
The brackets followed by a star means to repeat a \d, \s, or / an infinite amount of times.
The final \d$ means the ending must match a digit.

I have two problems, one of them is a regex

I am updating some code that I didn't write and part of it is a regex as follows:
\[url(?:\s*)\]www\.(.*?)\[/url(?:\s*)\]
I understand that .*? does a non-greedy match of everything in the second register.
What does ?:\s* in the first and third registers do?
Update: As requested, language is C# on .NET 3.5

The syntax (?:) is a way of putting parentheses around a subexpression without separately extracting that part of the string.
The author wanted to match the (.*?) part in the middle, and didn't want the spaces at the beginning or the end from getting in the way. Now you can use \1 or $1 (or whatever the appropriate method is in your particular language) to refer to the domain name, instead of the first chunk of spaces at the beginning of the string

?: makes the parentheses non-grouping. In that regex, you'll only pull out one piece of information, $1, which contains the middle (.*?) expression.

What does ?:\s* in the first and third registers do?
It's matching zero or more whitespace characters, without capturing them.
The regex author intends to allow trailing whitespace in the square-bracket-tags, matching all DNS labels following the "www." like so:
[url]www.foo.com[/url] # foo.com
[url ]www.foo.com[/url ] # same
[url ]www.foo.com[/url] # same
[url]www.foo.com[/url ] # same
Note that the regex also matches:
[url]www.[/url] # empty string!
and fails to match
[url]stackoverflow.com[/url] # no match, bummer

You may find this Regular Expressions Cheat Sheet very helpful (hopefully). I spent ages trying to learn Regex with no luck. And once I read this cheat-sheet - I immediately understood what I previously failed to learn.
http://krijnhoetmer.nl/stuff/regex/cheat-sheet/

Finding words strictly starting with $, Regex C#

I need to find all matches of word which strictly begins with "$" and contains only digits. So I wrote
[$]\d+
which gave me 4 matches for
$10 $10 $20a a$20
so I thought of using word boundaries using \b:
[$]\d+\b
But it again matched
a$20 for me.
I tried
\b[$]\d+\b
but I failed.
I'm looking for saying, ACCEPT ONLY IF THE WORD STARTS WITH $ and is followed by DIGITS. How do I tell IT STARTS WITH $, because I think \b is making it assume word boundaries which means surrounded inside alphanumeric characters.
What is the solution?

Not the best solution but this should work. (It does with your test case)
(?<=\s+|^)\$\d+\b

Have you tried
\B\$\d+\b

You were close, you just need to escape the $:
\B\$\d+\b
See the example matches here: http://regexhero.net/tester/?id=79d0ac3b-dd2c-4872-abb4-6a9780c91fc1

Try with ^\$\d+
where ^ denoted the beginning of a string.