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Can I use the same substring as part of different captures?

I want to create a function that will allow me to convert CamelCase to Title Case. This seems like a good task for regular expressions, but I am not committed to using regular expressions, if you have a better solution.
Here is my first attempt that works in most cases, but there are some issues I will get to in a few lines:
private static Regex camelSplitRegex = new Regex(#"(\S)([A-Z])");
private static String camelReplacement = "$1 $2";
public String SplitCamel(String text){
return camelSplitRegex.Replace(text, camelReplacement);
}
The regex pattern looks for a non-whitespace character (1st capture) followed by a capital letter (2nd capture). In the function, Regex.Replace is used to insert a space between the 1st and 2nd captures.
This works fine for many examples:
SplitCamel("privateField") returns "private Field"
SplitCamel("PublicMethod") returns "Public Method"
SplitCamel(" LeadingSpace") returns " Leading Space" without inserting an extra space before "Leading", as desired.
The problem I have is when dealing with multiple consecutive capital letters.
SplitCamel("NASA") returns "N AS A" not "N A S A"
SplitCamel("C3PO") returns "C3 PO" not "C3 P O"
SplitCamel("CAPS LOCK FEVER") returns "C AP S L OC K F EV E R" not "C A P S L O C K F E V E R"
In these cases, I believe the issue is that each capital letter is only being captured as either \S or [A-Z], but cannot be \S on one match and [A-Z] on the next match.
My main question is, "Does the .NET regex engine has some way of supporting the same substring being used as different captures on consecutive matches?" Secondarily, is there a better way of splitting camel case?

private static Regex camelSplitRegex = new Regex(#"(?<=\w)(?=[A-Z])");
private static String camelReplacement = " ";
does the job.
The problem with your pattern is that when you have the string "ABCD", \S matches A and ([A-Z]) matches B and you obtain "A BCD", but for the next replacement B is already consumed by the pattern and can't be used any more.
The way is to use lookarounds (a lookbehind (?<=...) and a lookahead (?=...)) that don't consume characters, they are only tests for the current position in the string, that's why you don't need any reference in the replacement string, you only need to put a space at the current position.
The \w character class contains unicode letters, unicode digits and the underscore. If you want to restrict the search to ASCII digits and letters, use [0-9a-zA-Z] instead.
To be more precise:
for unicode, use (?<=[\p{L}\p{N}])(?=\p{Lu}) that works with accented letters and other alphabets and digits.
for ASCII use (?<=[a-zA-Z0-9])(?=[A-Z])

Here's a non-regular expression way to do that.
public static string SplitCamel(this string stuff)
{
var builder = new StringBuilder();
char? prev = null;
foreach (char c in stuff)
{
if (prev.HasValue && !char.IsWhiteSpace(prev.Value) && 'A' <= c && c <= 'Z')
builder.Append(' ');
builder.Append(c);
prev = c;
}
return builder.ToString();
}
The following
Console.WriteLine("'{0}'", "privateField".SplitCamel());
Console.WriteLine("'{0}'", "PublicMethod".SplitCamel());
Console.WriteLine("'{0}'", " LeadingSpace".SplitCamel());
Console.WriteLine("'{0}'", "NASA".SplitCamel());
Console.WriteLine("'{0}'", "C3PO".SplitCamel());
Console.WriteLine("'{0}'", "CAPS LOCK FEVER".SplitCamel());
Prints
'private Field'
'Public Method'
' Leading Space'
'N A S A'
'C3 P O'
'C A P S L O C K F E V E R'

please consider switching to the value type string instead of the string class. Update to this.
private static Regex camelSplitRegex = new Regex(#"(^\S)?([A-Z])");

Regular expression to remove whitespace around a comma, except when quoted

I have a CSV file that has rows resembling this:
1, 4, 2, "PUBLIC, JOHN Q" ,ACTIVE , 1332
I am looking for a regular expression replacement that will match against these rows and spit out something resembling this:
1,4,2,"PUBLIC, JOHN Q",ACTIVE,1332
I thought this would be rather easy: I made the expression ([ \t]+,) and replaced it with ,. I made a complement expression (,[ \t]+) with a replacement of , and I thought I had achieved a good means of right-trimming and left-trimming strings.
...but then I noticed that my "PUBLIC, JOHN Q" was now "PUBLIC,JOHN Q" which isn't what I wanted. (Note the space following the comma is now gone).
What would be the appropriate expression to trim the white space before and after a comma, but leave quoted text untouched?
UPDATE
To clarify, I am using an application to handle the file. This application allows me to define multiple regular expression replacements; it does not provide a parsing capability. While this may not be the ideal mechanism for this, it would sure beat making another application for this one file.

If the engine used by your tool is the C# regular expression engine, then you can try the following expression:
(?<!,\s*"(?:[^\\"]|\\")*)\s+(?!(?:[^\\"]|\\")*"\s*,)
replace with empty string.
The guys answers assumed the quotes are balanced and used counting to determine if the space is part of a quoted value or not.
My expression looks for all spaces that are not part of a quoted value.
RegexHero Demo

Something like this might do the job:
(?<!(^[^"]*"[^"]*(("[^"]*){2})*))[\t ]*,[ \t]*
Which matches [\t ]*,[ \t]*, only when not preceded by an odd number of quotes.

Going with some CSV library or parsing the file yourself would be much more easier, and IMO should be preferable option here.
But if you really insist on a regex, you can use this one:
"\s+(?=([^\"]*\"[^\"]*\")*[^\"]*$)"
And replace it with empty string - ""
This regex matches one or more whitespaces, followed by an even number of quotes. This will of course work only if you have balanced quote.
(?x) # Ignore Whitespace
\s+ # One or more whitespace characters
(?= # Followed by
( # A group - This group captures even number of quotes
[^\"]* # Zero or more non-quote characters
\" # A quote
[^\"]* # Zero or more non-quote characters
\" # A quote
)* # Zero or more repetition of previous group
[^\"]* # Zero or more non-quote characters
$ # Till the end
) # Look-ahead end

string format(string val)
{
if (val.StartsWith("\"")) val = " " + val;
string[] vals = val.Split('\"');
for (int i = 0; i < vals.Length; i += 2) vals[i] = vals[i].Replace(" ", "").Replace("\t", "");
return string.Join("\t", vals);
}
This will work if you have properly closed quoted strings in between

Forget the regex (See Bart's comment on the question, regular expressions aren't suitable for CSV).
public static string ReduceSpaces( string input )
{
char[] a = input.ToCharArray();
int placeComma = 0, placeOther = 0;
bool inQuotes = false;
bool followedComma = true;
foreach( char c in a ) {
inQuotes ^= (c == '\"');
if (c == ' ') {
if (!followedComma)
a[placeOther++] = c;
}
else if (c == ',') {
a[placeComma++] = c;
placeOther = placeComma;
followedComma = true;
}
else {
a[placeOther++] = c;
placeComma = placeOther;
followedComma = false;
}
}
return new String(a, 0, placeComma);
}
Demo: http://ideone.com/NEKm09

Regex Failing "Unrecognized Escape sequence"

module107 should be matching the sample text Module ID="107"
Can you help me understand where I am going wrong in the code?
var module107 = Regex("\A*Module\sID=\"107\"");
ERROR: Unrecognized escape sequence

The problem here is, you want to escape for two different levels. The \A is a escape sequence for the regex. But the issue is, there is at first the string that tries to interpret escape sequences and the string does not know the escape sequence \A or \s (I don't know).
Two solutions are possible:
if you are escaping for the regex, double the \. So
var module107 = Regex("\\A*Module\\sID=\"107\"");
is the string and after the string is processed, the regex is \A*Module\sID="107"
Use verbatim strings. If you add a # before the string, escape sequences are not evaluated by the string. So Regex(#"\A*Module\sID=") would end as regex \A*Module\sID=
But now you are getting problems with the " you want to have in the regex. You can add a " to a verbatim string by doubling it:
var module107 = Regex(#"\A*Module\sID=""107""");

Description
This will match the module id="107" where the number is any quantity of digits surrounded by double quotes. I've changed your escaped quotes with [""] so they can be nested into a string. I'm using \b which will look for the word break and will allow the string to appear anywhere in the input. But if you're looking to validate a specific text, then you can do the \A or ^ to denote the beginning of the string instead.
\b(Module\s+ID=[""](\d{1,})[""])
Groups
Group 0 will capture the entire string
will get the have from Module to the second quote
will get the value inside the quotes
C# Code Example:
using System;
using System.Text.RegularExpressions;
namespace myapp
{
class Class1
{
static void Main(string[] args)
{
String sourcestring = "for Module ID=""107"" Can you h";
Regex re = new Regex(#"\b(Module\s+ID=[""](\d{1,})[""])",RegexOptions.IgnoreCase);
MatchCollection mc = re.Matches(sourcestring);
int mIdx=0;
foreach (Match m in mc)
{
for (int gIdx = 0; gIdx < m.Groups.Count; gIdx++)
{
Console.WriteLine("[{0}][{1}] = {2}", mIdx, re.GetGroupNames()[gIdx], m.Groups[gIdx].Value);
}
mIdx++;
}
}
}
}
$matches Array:
(
[0] => Array
(
[0] => Module ID="107"
)
[1] => Array
(
[0] => Module ID="107"
)
[2] => Array
(
[0] => 107
)
)

The key thing is that the text you've typed is interpreted as a string first, then as a Regex. The string interpretation also looks at '\'s and uses them in its interpretation.
As Tyanna says, this means you need to escape those '\'s so that they don't get 'used up' as the string is read or confuse the string parser.
An alternative approach you might like to try is to use a string literal. This can be a bit cleaner when working with Regexes, as you don't end up with lots of slashes (just more double-quotes sometimes):
var module107 = new Regex(#"\A*Module\sID=""107""");

How to grab specific elements out of a string

I need to be able to grab specific elements out of a string that start and end with curly brackets. If I had a string:
"asjfaieprnv{1}oiuwehern{0}oaiwefn"
How could I grab just the 1 followed by the 0.

Regex is very useful for this.
What you want to match is:
\{ # a curly bracket
# - we need to escape this with \ as it is a special character in regex
[^}] # then anything that is not a curly bracket
# - this is a 'negated character class'
+ # (at least one time)
\} # then a closing curly bracket
# - this also needs to be escaped as it is special
We can collapse this to one line:
\{[^}]+\}
Next, you can capture and extract the inner contents by surrounding the part you want to extract with parentheses to form a group:
\{([^}]+)\}
In C# you'd do:
var matches = Regex.Matches(input, #"\{([^}]+)\}");
foreach (Match match in matches)
{
var groupContents = match.Groups[1].Value;
}
Group 0 is the whole match (in this case including the { and }), group 1 the first parenthesized part, and so on.
A full example:
var input = "asjfaieprnv{1}oiuwehern{0}oaiwef";
var matches = Regex.Matches(input, #"\{([^}]+)\}");
foreach (Match match in matches)
{
var groupContents = match.Groups[1].Value;
Console.WriteLine(groupContents);
}
Outputs:
1
0

Use the Indexof method:
int openBracePos = yourstring.Indexof ("{");
int closeBracePos = yourstring.Indexof ("}");
string stringIWant = yourstring.Substring(openBracePos, yourstring.Len() - closeBracePos + 1);
That will get your first occurrence. You need to slice your string so that the first occurrence is no longer there, then repeat the above procedure to find your 2nd occurrence:
yourstring = yourstring.Substring(closeBracePos + 1);
Note: You MAY need to escape the curly braces: "{" - not sure about this; have never dealt with them in C#

This looks like a job for regular expressions
using System.Text.RegularExpressions;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
string str = "asjfaieprnv{1}oiuwe{}hern{0}oaiwefn";
Regex regex = new Regex(#"\{(.*?)\}");
foreach( Match match in regex.Matches(str))
{
Console.WriteLine(match.Groups[1].Value);
}
}
}
}

Formatting sentences in a string using C#

I have a string with multiple sentences. How do I Capitalize the first letter of first word in every sentence. Something like paragraph formatting in word.
eg ."this is some code. the code is in C#. "
The ouput must be "This is some code. The code is in C#".
one way would be to split the string based on '.' and then capitalize the first letter and then rejoin.
Is there a better solution?

In my opinion, when it comes to potentially complex rules-based string matching and replacing - you can't get much better than a Regex-based solution (despite the fact that they are so hard to read!). This offers the best performance and memory efficiency, in my opinion - you'll be surprised at just how fast this'll be.
I'd use the Regex.Replace overload that accepts an input string, regex pattern and a MatchEvaluator delegate. A MatchEvaluator is a function that accepts a Match object as input and returns a string replacement.
Here's the code:
public static string Capitalise(string input)
{
//now the first character
return Regex.Replace(input, #"(?<=(^|[.;:])\s*)[a-z]",
(match) => { return match.Value.ToUpper(); });
}
The regex uses the (?<=) construct (zero-width positive lookbehind) to restrict captures only to a-z characters preceded by the start of the string, or the punctuation marks you want. In the [.;:] bit you can add the extra ones you want (e.g. [.;:?."] to add ? and " characters.
This means, also, that your MatchEvaluator doesn't have to do any unnecessary string joining (which you want to avoid for performance reasons).
All the other stuff mentioned by one of the other answerers about using the RegexOptions.Compiled is also relevant from a performance point of view. The static Regex.Replace method does offer very similar performance benefits, though (there's just an additional dictionary lookup).
Like I say - I'll be surprised if any of the other non-regex solutions here will work better and be as fast.
EDIT
Have put this solution up against Ahmad's as he quite rightly pointed out that a look-around might be less efficient than doing it his way.
Here's the crude benchmark I did:
public string LowerCaseLipsum
{
get
{
//went to lipsum.com and generated 10 paragraphs of lipsum
//which I then initialised into the backing field with #"[lipsumtext]".ToLower()
return _lowerCaseLipsum;
}
}
[TestMethod]
public void CapitaliseAhmadsWay()
{
List<string> results = new List<string>();
DateTime start = DateTime.Now;
Regex r = new Regex(#"(^|\p{P}\s+)(\w+)", RegexOptions.Compiled);
for (int f = 0; f < 1000; f++)
{
results.Add(r.Replace(LowerCaseLipsum, m => m.Groups[1].Value
+ m.Groups[2].Value.Substring(0, 1).ToUpper()
+ m.Groups[2].Value.Substring(1)));
}
TimeSpan duration = DateTime.Now - start;
Console.WriteLine("Operation took {0} seconds", duration.TotalSeconds);
}
[TestMethod]
public void CapitaliseLookAroundWay()
{
List<string> results = new List<string>();
DateTime start = DateTime.Now;
Regex r = new Regex(#"(?<=(^|[.;:])\s*)[a-z]", RegexOptions.Compiled);
for (int f = 0; f < 1000; f++)
{
results.Add(r.Replace(LowerCaseLipsum, m => m.Value.ToUpper()));
}
TimeSpan duration = DateTime.Now - start;
Console.WriteLine("Operation took {0} seconds", duration.TotalSeconds);
}
In a release build, the my solution was about 12% faster than the Ahmad's (1.48 seconds as opposed to 1.68 seconds).
Interestingly, however, if it was done through the static Regex.Replace method, both were about 80% slower, and my solution was slower than Ahmad's.

Here's a regex solution that uses the punctuation category to avoid having to specify .!?" etc. although you should certainly check if it covers your needs or set them explicitly. Read up on the "P" category under the "Supported Unicode General Categories" section located on the MSDN Character Classes page.
string input = #"this is some code. the code is in C#? it's great! In ""quotes."" after quotes.";
string pattern = #"(^|\p{P}\s+)(\w+)";
// compiled for performance (might want to benchmark it for your loop)
Regex rx = new Regex(pattern, RegexOptions.Compiled);
string result = rx.Replace(input, m => m.Groups[1].Value
+ m.Groups[2].Value.Substring(0, 1).ToUpper()
+ m.Groups[2].Value.Substring(1));
If you decide not to use the \p{P} class you would have to specify the characters yourself, similar to:
string pattern = #"(^|[.?!""]\s+)(\w+)";
EDIT: below is an updated example to demonstrate 3 patterns. The first shows how all punctuations affect casing. The second shows how to pick and choose certain punctuation categories by using class subtraction. It uses all punctuations while removing specific punctuation groups. The third is similar to the 2nd but using different groups.
The MSDN link doesn't spell out what some of the punctuation categories refer to, so here's a breakdown:
P: all punctuations (comprises all of the categories below)
Pc: underscore _
Pd: dash -
Ps: open parenthesis, brackets and braces ( [ {
Pe: closing parenthesis, brackets and braces ) ] }
Pi: initial single/double quotes (MSDN says it "may behave like Ps/Pe depending on usage")
Pf: final single/double quotes (MSDN Pi note applies)
Po: other punctuation such as commas, colons, semi-colons and slashes ,, :, ;, \, /
Carefully compare how the results are affected by these groups. This should grant you a great degree of flexibility. If this doesn't seem desirable then you may use specific characters in a character class as shown earlier.
string input = #"foo ( parens ) bar { braces } foo [ brackets ] bar. single ' quote & "" double "" quote.
dash - test. Connector _ test. Comma, test. Semicolon; test. Colon: test. Slash / test. Slash \ test.";
string[] patterns = {
#"(^|\p{P}\s+)(\w+)", // all punctuation chars
#"(^|[\p{P}-[\p{Pc}\p{Pd}\p{Ps}\p{Pe}]]\s+)(\w+)", // all punctuation chars except Pc/Pd/Ps/Pe
#"(^|[\p{P}-[\p{Po}]]\s+)(\w+)" // all punctuation chars except Po
};
// compiled for performance (might want to benchmark it for your loop)
foreach (string pattern in patterns)
{
Console.WriteLine("*** Current pattern: {0}", pattern);
string result = Regex.Replace(input, pattern,
m => m.Groups[1].Value
+ m.Groups[2].Value.Substring(0, 1).ToUpper()
+ m.Groups[2].Value.Substring(1));
Console.WriteLine(result);
Console.WriteLine();
}
Notice that "Dash" is not capitalized using the last pattern and it's on a new line. One way to make it capitalized is to use the RegexOptions.Multiline option. Try the above snippet with that to see if it meets your desired result.
Also, for the sake of example, I didn't use RegexOptions.Compiled in the above loop. To use both options OR them together: RegexOptions.Compiled | RegexOptions.Multiline.

You have a few different options:
Your approach of splitting the string, capitalizing and then re-joining
Using regular expressions to perform a replace of the expressions (which can be a bit tricky for case)
Write a C# iterator that iterates over each character and yields a new IEnumerable<char> with the first letter after a period in upper case. May offer benefit of a streaming solution.
Loop over each char and upper-case those that appear immediately after a period (whitespace ignored) - a StringBuffer may make this easier.
The code below uses an iterator:
public static string ToSentenceCase( string someString )
{
var sb = new StringBuilder( someString.Length );
bool wasPeriodLastSeen = true; // We want first letter to be capitalized
foreach( var c in someString )
{
if( wasPeriodLastSeen && !c.IsWhiteSpace )
{
sb.Append( c.ToUpper() );
wasPeriodLastSeen = false;
}
else
{
if( c == '.' ) // you may want to expand this to other punctuation
wasPeriodLastSeen = true;
sb.Append( c );
}
}
return sb.ToString();
}

I don't know why, but I decided to give yield return a try, based on what LBushkin had suggested. Just for fun.
static IEnumerable<char> CapitalLetters(string sentence)
{
//capitalize first letter
bool capitalize = true;
char lastLetter;
for (int i = 0; i < sentence.Length; i++)
{
lastLetter = sentence[i];
yield return (capitalize) ? Char.ToUpper(sentence[i]) : sentence[i];
if (Char.IsWhiteSpace(lastLetter) && capitalize == true)
continue;
capitalize = false;
if (lastLetter == '.' || lastLetter == '!') //etc
capitalize = true;
}
}
To use it:
string sentence = new String(CapitalLetters("this is some code. the code is in C#.").ToArray());

Do your work in a StringBuffer.
Lowercase the whole thing.
Loop through and uppercase leading chars.
Call ToString.