Develop Reference

How to match string by using regular expression which will not allow same special character at same time?

I m trying to matching a string which will not allow same special character at same time
my regular expression is:
[RegularExpression(#"^+[a-zA-Z0-9]+[a-zA-Z0-9.&' '-]+[a-zA-Z0-9]$")]
this solve my all requirement except the below two issues
this is my string : bracks
acceptable :
bra-cks, b-r-a-c-ks, b.r.a.c.ks, bra cks (by the way above regular expression solved this)
not acceptable:
issue 1: b.. or bra..cks, b..racks, bra...cks (two or more any special character together),
issue 2: bra cks (two ore more white space together)

You can use a negative lookahead to invalidate strings containing two consecutive special characters:
^(?!.*[.&' -]{2})[a-zA-Z0-9.&' -]+$
Demo: https://regex101.com/r/7j14bu/1

The goal
From what i can tell by your description and pattern, you are trying to match text, which start and end with alphanumeric (due to ^+[a-zA-Z0-9] and [a-zA-Z0-9]$ inyour original pattern), and inside, you just don't want to have any two consecuive (adjacent) special characters, which, again, guessing from the regex, are . & ' -
What was wrong
^+ - i think here you wanted to assure that match starts at the beginning of the line/string, so you don't need + here
[a-zA-Z0-9.&' '-] - in this character class you doubled ' which is totally unnecessary
Solution
Please try pattern
^[a-zA-Z0-9](?:(?![.& '-]{2,})[a-zA-Z0-9.& '-])*[a-zA-Z0-9]$
Pattern explanation
^ - anchor, match the beginning of the string
[a-zA-Z0-9] - character class, match one of the characters inside []
(?:...) - non capturing group
(?!...) - negative lookahead
[.& '-]{2,} - match 2 or more of characters inside character class
[a-zA-Z0-9.& '-] - character class, match one of the characters inside []
* - match zero or more text matching preceeding pattern
$ - anchor, match the end of the string
Regex demo

Some remarks on your current regex:
It looks like you placed the + quantifiers before the pattern you wanted to quantify, instead of after. For instance, ^+ doesn't make much sense, since ^ is just the start of the input, and most regex engines would not even allow that.
The pattern [a-zA-Z0-9.&' '-]+ doesn't distinguish between alphanumerical and other characters, while you want the rules for them to be different. Especially for the other characters you don't want them to repeat, so that + is not desired for those.
In a character class it doesn't make sense to repeat the same character, like you have a repeat of a quote ('). Maybe you wanted to somehow delimit the space, but realise that those quotes are interpreted literally. So probably you should just remove them. Or if you intended to allow for a quote, only list it once.
Here is a correction (add the quote if you still need it):
^[a-zA-Z0-9]+(?:[.& -][a-zA-Z0-9]+)*$
Follow-up
Based on a comment, I suspect you would allow a non-alphanumerical character to be surrounded by single spaces, even if that gives a sequence of more than one non-alphanumerical character. In that case use this:
^[a-zA-Z0-9]+(?:(?:[ ]|[ ]?[.&-][ ]?)[a-zA-Z0-9]+)*$
So here the space gets a different role: it can optionally occur before and after a delimiter (one of ".&-"), or it can occur on its own. The brackets around the spaces are not needed, but I used them to stress that the space is intended and not a typo.

Regex for first name

I am quite new to regex thing and need regex for first name which satisfies following conditions:
First Name must contain letters only. It may contain spaces, hyphens, or apostrophes.
It must begin with letters.
All other characters and numbers are not valid.
Special characters ‘ and – cannot be together (e.g. John’-s is not allowed)
An alphabet should be present before and after the special characters ‘ and – (e.g. John ‘s is not allowed)
Two consecutive spaces are not allowed (e.g. Annia St is not allowed)
Can anyone help? I tried this ^([a-z]+['-]?[ ]?|[a-z]+['-]?)*?[a-z]$ but it's not working as expected.

Regexes are notoriously difficult to write and maintain.
One technique that I've used over the years is to annotate my regexes by using named capture groups. It's not perfect, but can greatly help with the readability and maintainability of your regex.
Here is a regex that meets your requirements.
^(?<firstchar>(?=[A-Za-z]))((?<alphachars>[A-Za-z])|(?<specialchars>[A-Za-z]['-](?=[A-Za-z]))|(?<spaces> (?=[A-Za-z])))*$
It is split down into the following parts:
1) (?<firstchar>(?=[A-Za-z])) This ensures the first character is an alpha character, upper or lowercase.
2) (?<alphachars>[A-Za-z]) We allow more alpha chars.
3) (?<specialchars>[A-Za-z]['-](?=[A-Za-z])) We allow special characters, but only with an alpha character before and after.
4) (?<spaces> (?=[A-Za-z])) We allow spaces, but only one space, which must be followed by alpha characters.
You should use a testing tool when writing regexes, I'd recommend https://regex101.com/
You can see from the screenshot below how this regex performs.
Take the regex I've given you, run it in https://regex101.com/ with samples you'd like to match against, and tweak it to fit your requirements. Hopefully I've given you enough information to be self sufficient in customising it to your needs.
You can use this link to run the regex https://regex101.com/r/O2wFfi/1/
Edit
I've updated to address the issue in your comment, rather than just give you the code, I will explain the problem and how I fixed it.
For your example "Sam D'Joe", if we run the original regex, the following happens.
^(?<firstchar>[A-Za-z])((?<alphachars>[A-Za-z])|(?<specialchars>[A-Za-z]['-][A-Za-z])|(?<spaces> [A-Za-z]))*$
1) ^ matches the start of the string
2) (?<firstchar>[A-Za-z]) matches the first character
3) (?<alphachars>[A-Za-z]) matches every character up to the space
4) (?<spaces> [A-Za-z]) matches the space and the subsequent alpha char
Matches consume the characters that they match
This is where we run into a problem. Our "specialchars" part of the regex matches an alpha char, our special char and then another alpha char ((?<specialchars>[A-Za-z]['-](?=[A-Za-z]))).
The thing you need to know about regexes, is each time you match a character, that character is then consumed. We've already matched the alpha char before the special character, so our regex will never match.
Each step actually looks like this:
1) ^ matches the start of the string
2) (?<firstchar>[A-Za-z]) matches the first character
3) (?<alphachars>[A-Za-z]) matches every character up to the space
4) (?<spaces> [A-Za-z]) matches the space and the subsequent alpha char
and then we're left with the following
We cannot match this, because one of our rules is "An alphabet should be present before and after the special characters ‘ and –".
Lookahead
Regex has a concept called "lookahead". A lookahead allows you to match a character without consuming it!
The syntax for a lookahead is ?= followed by what you want to match. E.g. ?=[A-Z] would look ahead for a single character that is an uppercase letter.
We can fix our regex, by using lookaheads.
1) ^ matches the start of the string
2) (?<firstchar>[A-Za-z]) matches the first character
3) (?<alphachars>[A-Za-z]) matches every character up to the space
4) We now change our "spaces" regex, to lookahead to the alpha char, so we don't consume it. We change (?<spaces> [A-Za-z]) to (?<spaces> ?=[A-Za-z]). This matches the space and looks ahead to the subsequent alpha char, but doesn't consume it.
5) (?<specialchars>[A-Za-z]['-][A-Za-z]) matches the alpha char, the special char, and the subsequent alpha char.
6) We use a wildcard to repeat matching our previous 3 rules multiple times, and we match until the end of the line.
I also added lookaheads to the "firstchar", "specialchars" and "spaces" capture groups, I've bolded the changes below.
^(?<firstchar>(?=[A-Za-z]))((?<alphachars>[A-Za-z])|(?<specialchars>[A-Za-z]['-](?=[A-Za-z]))|(?<spaces> (?=[A-Za-z])))*$

This short regex should do it ^([a-zA-Z]+?)([-\s'][a-zA-Z]+)*?$ ,
([a-zA-Z]+?) - Means the String should start with alphabets.
([-\s'][a-zA-Z]+)*? - Means the string must have hyphen,space or apostrophe followed by alphabets.
^ and $ - denote start and end of string
Here's the link to regex demo.

Try this one
^[^- '](?=(?![A-Z]?[A-Z]))(?=(?![a-z]+[A-Z]))(?=(?!.*[A-Z][A-Z]))(?=(?!.*[- '][- '.]))(?=(?!.*[.][-'.]))[A-Za-z- '.]{2,}$
Demo

.Net Regex - last of repeating characters

I'm trying to capture everything inside curly bracers, but in some cases there may be multiple bracers and I want the external ones.
For example: I want to capture {{this}} part
I'll need {{this}} as the capture.
So I went with ({[^}]+}+) to capture the inner text, but of course this will yield multiple captures {{this} and {{this}}.
So I tried telling the regex to search for the phrase but only if the next character is not curly bracers: ({[^}]+}+)[^}]. This works, unless the capture is at the end of the input, in which case it doesn't work cause it expects a non } character at the end.
So I tried adding end of string option ({[^}]+}+)[$|^}], but for some reason, this will capture {{this} again. I have no idea why, it should only capture if the next char is end of input or not curly bracers...
Suggestions?
Edit:
Just to be clear, I'm not searching for valid nested parenthesis, only for text between { and the first matching } (no nesting!), however there may be cases where instead of one open/close brace there are two (so {something} and {{something}} both need to be caught).
The reason for this, is that the original text always has double braces {{ }}, but sometimes before the regex the text undergoes string.Format, in which case the double braces become single braces.

Generally, regex is not powerful enough to do this. However, .NET regex engine supports so-called Atomic Grouping, which let you process groups with balanced parentheses:
{(?>{(?<DEPTH>)|}(?<-DEPTH>)|[^}]+)*}(?(DEPTH)(?!))

If you want to match all text between braces, I think this should do the trick:
{+.*?}+
This matches everything between braces, taking all consecutive braces and as few internal characters as possible.
Further explanation: matches 1 or more { ({+), then any amount of any character (.*) but gives you the shortest string that does it (?), and finally matches 1+ } (}+). Without that ?, if you have {a} {b} it would match the whole thing instead of {a} and {b} separately.
If you won't want spaces between the braces, you can use this:
{+\S*?}+
If you only want letters, use \w instead of \S.
The only thing this is not validating is that the same amount of braces are used. Do you need that?
Result comparison (should be a comment).
Considering {{{{{{this}}}}}Blabla, I get this:
Regex author: c0d3rman
Matched string: {{{{{{this}}}}}B
Groups: 2 ({{{{{{this}}}}}B and {{{{{{this}}}}})
Captures: {{{{{{this}}}}}
Regex author: dasblinkenlight
Matched string: {{{{{this}}}}}
Groups: 2 ({{{{{this}}}}} and {})
Captures: {{{{{this}}}}}
Note: symmetric braces
Regex author: Andrew
Matched string: {{{{{{this}}}}}
Groups: {{{{{{this}}}}}
Captures: {{{{{{this}}}}}

You seem to have used a character class at the end instead of a non-capturing group. Try:
({[^}]+}+)(?:$|[^}])
This is a very small modification to your final attempt, that just uses correct syntax. In your final attempt you have [$|^}]. The issue with this is that you can't have an or | inside a character class []. Most special characters are escaped inside a character class, with a couple exceptions, one of which is ^ if it is the first character. So [$|^}] means any of the four literal characters $, |, ^, or }. What I did is change the syntax to what you intended by using a non-capturing group (?:stuff) this group does not save its contents and is purely for grouping. As such (?:$|[^}]) means an end-of-line or a non-}, as you wanted.
Note that this makes no effort to balance the curly braces (match the number of braces at the beginning and end).

RegEx : Find match based on 1st two chars

I am new to RegEx and thus have a question on RegEx. I am writing my code in C# and need to come up with a regex to find matching strings.
The possible combination of strings i get are,
XYZF44DT508755
ABZF44DT508755
PQZF44DT508755
So what i need to check is whether the string starts with XY or AB or PQ.
I came up with this one and it doesn't work.
^((XY|AB|PQ).){2}
Note: I don't want to use regular string StartsWith()
UPDATE:
Now if i want to try a new matching condition like this -
If string starts with "XY" or "AB" or "PQ" and 3rd character is "Z" and 4th character is "F"
How to write the RegEx for that?

You can modify you expression to the following and use the IsMatch() method.
Regex.IsMatch(input, "^(?:XY|AB|PQ)")
The outer capturing group in conjuction with . (any single character) is trying to match a third character and then repeat the sequence twice because of the range quantifier {2} ...
According to your updated edit, you can simply place "ZF" after the grouping construct.
Regex.IsMatch(input, "^(?:XY|AB|PQ)ZF")

You want to test for just ^(XY|AB|PQ). Your RegEx means: Search for either XY, AB or PQ, then a random character, and repeat the whole sequence twice, for example "XYKPQL" would match your RegEx.
This is a screenshot of the matches on regex101:
^ forces the start of line,
(...) creates a matching group and
XY|AB|PQ matches either XY, AB or PQ.
If you want the next two characters to be ZF, just append ZF to the RegEx so it becomes ^(XY|AB|PQ)ZF.
Check out regex101, a great way to test your RegExes.

You were on the right track. ^(XY|AB|PQ) should match your string correctly.
The problem with ^((XY|AB|PQ).){2} is following the entire group with {2}. This means exactly 2 occurrences. That would be 2 occurrences of your first 2 characters, plus . (any single character), meaning this would match strings like XY_AB_. The _ could be anything.
It may have been your intention with the . to match a larger string. In this case you might try something along the lines of ^((XY|AB|PQ)\w*). The \w* will match 0 or more occurrences of "word characters", so this should match all of XYZF44DT508755 up to a space, line break, punctuation, etc., and not just the XY at the beginning.
There are some good tools out there for understanding regexes, one of my favorites is debuggex.
UPDATE
To answer your updated question:
If string starts with "XY" or "AB" or "PQ" and 3rd character is "Z" and 4th character is "F"
The regex would be (assuming you want to match the entire "word").
^((XY|AB|PQ)ZF\w*)
Debuggex Demo

Are these the proper regex expressions

I am trying to make a few regex strings to use in my syntax highlighter, this if the first time I have ever used them and I am having a deal of difficulty...
The first four are, I will have a specified character followed by any number of numbers, match it.
The best I have is "G[0-9]|G[0-9][0-9]|G[0-9][0-9][0-9]" to match either G#, G##, or G###
but I want to do G with any number of numbers after it.
The next three are, I will have a character (X,Y,Z, or P) and I want to match it if there is no letter or symbol behind it
"[X|Y|Z|P][0-9]"
These next few are harder, match "#11.11=11.11" where 1 is a number and there can be any number of numbers between the pound sign, the periods, and the equal sign. And the periods do not have to be there can also be "#11=11" or " #1.1=11" or "#11=1.1"
I have no idea... "#[0-9][ |.] ..."
Anything after a " ' " and between a newline
'[A-Za-z0-9]\n" but I know this only gives me one character...
And the easy one I think is anything between two () or []
"(*) | [*]"?

Quick and dirty, but tested using regexpal
1) G[0-9]{1-3} - the '{1-3}' specifies the last symbol to occur one to three times.
2) ((.*|)) - you put a '\' before the '(' and ')' as escape characters
3) [0-9]1*(.|)1*=1*(.|)1 - this matches your three examples
4) \'.*\n - I think this should work... '\n' represents a new line char right?
5) ((|[).*()|]) - this one has those escape characters again
Again...quick and dirty. Regexpal.com is your friend

1> G[0-9]{1,3}
2> No, it's WRONG.
The correct one is [XYZ][0-9]
(you do not use an OR operator (|), but just write the characters side by side within square braces)

You should really look up how to use regexes. Having said that:
I will have a specified character followed by any number of numbers, match it
G\d+
I will have a character (X,Y,Z, or P) and I want to match it if there
is no letter or symbol behind it
(?<!\w)[XYZP][0-9]
These next few are harder, make "#11.11=11.11" blue
Huh?
Anything after a " ' " and between a newline
'(.+?)\n
And the easy one I think is anything between two () or []
\(.+?\)|\[.+?\]

And the easy one I think is anything between two () or []
"(*) | [*]"?
#"\([^(]*\)" and #"\[[^\[]*\]"
It means: an open bracket - then any number of characters which are not an open bracket - and a close bracket.
Slashes are required to indicate to the regex engine that brackets should be treated literally.
# - verbatim string - is to inform C#, in turn, that those slashes should be taken literally and not as C# escape characters.
Anything after a " ' " and between a newline
Similarly: #"'[^']*\n"

G\d+
[XYZP](?=\d)
#(\d+(\.\d+)?)=(\d+(\.\d+)?)
'.*?\n
\(.*?\)|\[.*?\]
Regex explanation here.

The first one:
G[0-9]+
In regular expressions + means at least 1 or more (repetitions of the previous "characters").
You could also use * for none or any number of repetitions.
The second might be something like this:
^(X|Y|Z|P)$
This actually matches only if it's at the beginning of a line and has no characters behind. If you want it to be anywhere and only exclude certain characters behind it, modify the following:
[XYZP][^0-9a-z]
This is X or Y or Z or P followed by NOT 0-9 and NOT a-z
Notice that I use the OR character | in the first example in brackets, but not in the square brackets.
For the third one:
#[0-9]+\.[0-9]+=[0-9]+\.[0-9]+
Might not be 100 percent correct, I always confuse when to escape which characters. You might need to escape # and =.
For the last one:
(\(.*\)|\[.*\])

For the first one you can use this Regex :
^G\d+

For G with any number of digits after it
\b([Gg]\d+)\b
This matches a wordboundary (\b) followed by a lower or upper G [Gg], followed by 1 or more (+) digits (\d), followed by a wordboundary (\b)
The next three are, I will have a character (X,Y,Z, or P) and I want
to match it if there is no letter or symbol behind it
This is a little tougher
\b[XYZP]([\W]|_)
This matches an XYZ or P followed by a non-word character \W, (word characters are typically a-z, 0-9 and the underscore), so after saying we don't want a word character, we add in that the _ is allowed.

I use perl for regex, but it should hopefully be the same as what you're looking for.
For the first one, G[0-9]+ should work. The square brackets means that the regex looks for only one of the characters within the brackets (the characters being 0 through 9) and the + right after it means that it looks for one or more matches.
The second is a bit more tricky, but I would use \s[XYPZ]. The square brackets function the same as before, only matching one of X, Y, P or Z. Also the \s matches any whitespace character (tab, space, newline, etc.).
For the third one, I would try #[0-9]+\.?[0-9]+=[0-9]+\.?[0-9]+. If we go from left to right, we encounter \.? and it's new. \. matches a literal period (you have to escape it with the backslash, as just a period by itself means that it can match one of any character). The question mark means that the period can either be there or not (matches zero or one instance of a period).
The fourth one: '.*\n. The combination of the period by itself and the asterisk means that it'll match zero or more characters, the characters being anything at all. I'm not too sure if you need to escape the single quotes though.
And for the fifth one, (\(.*\)|\[.*\]) should do the trick. You need to escape the []() inside the brackets because they mean something by themselves. Also, the | means or, so the regex can either matches whatever is on the left side of the bar, or on the right.

You can specify repetitions in different ways. A star "*" after a term means, repeat the term zero, one or several times. A plus sign "+" means, repeat the term one or several times. You can also specify a number range with {n,m}. In your case the expression would be
G\d{1-3}
where \d is a digit.
With this expression you can match a position that does not preceed a suffix
find(?!suffix)
I am not sure what you mean by symbol
[XYZP](?![a-zA-Z specify your symbols here])
For the pound number
\#\d+(\.\d+)?=\d+(\.\d+)?
\# the pound sign
\d+ at least one digit
(\.\d+)? optionally (?) a period succeeded by at least one digit
finally an equal sign succeeded by another number
Everything between "'" and \n. Use this pattern here, which finds a position between a prefix and a suffix.
(?<=prefix)find(?=suffix)
(?<=').*(?=\n)
.* means any character as many times as possible. Alternatively you could use
(?<=').*?(?=\n)
.* means any character as few times as possible, if too many \n are taken. Also be carefult with the RegexOption.Multiline. Depending on its setting you will have to test for the end of line with $ instead of \n.
For the parentheses () or [] you can use the same pattern again
(?<=prefix)find(?=suffix)
(?<=\().*?(?=\))|(?<=\[).*?(?=])
where | is the alternative.