Replace string but only for particular pattern in c#

Question

I have 2 strings, one is main string and other is pattern string to be replaced. I want to replace the some part of main string with pattern string only if particular pattern is matched in main string.

Example:

string mainstring = "[def].[ijk] = [abc].[def].[ijk]";
string pattern =    "[lmn].[def].[ijk]";

i want final string as

[lmn].[def].[ijk] = [abc].[def].[ijk]

i.e. if only 2 part is there in string than only replace not for 3 parts

i am using:

mainstring = mainstring.Replace("[def].[ijk]",pattern);

but it replaces as,

[lmn].[def].[ijk] = [abc].[lmn].[def].[ijk]
                          ^-------+-------^
                                  |
                                  +-- don't replace here

but I want as

[lmn].[def].[ijk] = [abc].[def].[ijk]

EDIT: Additional rule for the replacement:

You can touch left hand side or right hand side but the pattern should be alone without anything at before or after.

Mong Zhu · Accepted Answer · 2017-08-02 09:42:41Z

1

I am trying to follow this rule:

i.e. if only 2 part is there in string than only replace not for 3 parts

(interpreting it as replace only if the matching pattern is not inbetween other parts, or: neither preceded nor follwed by a .)

This regular expression should satisfy this rule above:

string mainstring = "[def].[ijk] = [abc].[def].[ijk]";
// will replace only right side
//string mainstring = "[abc].[def].[ijk] = [def].[ijk]"; 

string replacement = "[lmn].[def].[ijk]";

string pattern = @"(?<!\.)\[def\].\[ijk\](?!\.)";

string output = Regex.Replace(mainstring, pattern, replacement);

EDIT:

if you need to transform you basic pattern into a regex format replace both parentheses:

string searchPattern  = "[def].[ijk]";

searchPattern = searchPattern.Replace("[", @"\[").Replace("]", @"\]");

and place the result in between the look ahead and look behind pattern:

string pattern = @"(?<!\.)" + searchPattern + @"(?!\.)";

Explanation:

(?<!\.): the pattern is called negative lookahead. It will match only if the preceding character is not a . (dot)

(?!\.): the pattern is called negative lookbehind. It will match only if the following character is not a . (dot)

the replacement of [ ] to \[ \] is necessary because regex treats the parentheses as an interval or group and matches every character between the parentheses. Like [0-9] will match any digit between 0 and 9. But if you use an escape sequence @"\[0-9\]" it will take the parentheses as a character and match the pattern "[0-9]". I hope it became a little clearer

edited Aug 2, 2017 at 9:42

answered Aug 2, 2017 at 6:20

Mong Zhu

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13 Comments

Arc Over a year ago

i can split the problem is the split character is not fix. i.e. it can have =,!=, <,>,<> and other operators. so using particular character string wont help also it is not fixed which part should be replaced, i.e. in example you r replacing on [0]th index, but my pattern can be on any index

Mong Zhu Over a year ago

@Arc then you really should specify exactly how the rules are for replacement. If there is no clear rule, or at least a couple of contraints this will be very difficult to solve.

Arc Over a year ago

Following are some rules,to be followed : 1. The split characters are not fixed, any character can be there (like =,!=,<,>,<>) 2. the patterns position is not fixed, it can be either left or right side of split character. And split characters are only above specified, it can be +,-,*,/,&,(,),And, Or, Not,=,<>,<,<=,>=,>. also all the text functions, date function, conversion function, text function, summary functions, logical operators

Mong Zhu Over a year ago

@Arc which rule determines that only the left handside in your example should be replaced but not the right handside?

Arc Over a year ago

That is what i am saying, it can be on any side, either left or right side :(

|

Tommy · Accepted Answer · 2017-08-02 07:28:48Z

0

does this work for you?

 string mainstring = "[def].[ijk] = [abc].[def].[ijk]";

            string leftFillWith = "[lmn].[def].[ijk]";

            string pattern = @"((\[lmn\]).)?((\[def\]).)?(\[ijk\])? =";

          var expected=  Regex.Replace(mainstring, pattern,match=> leftFillWith+" =" );

if only two parts are accepted:

  string mainstring = "[def].[ijk] = [abc].[def].[ijk]";

            string leftFillWith = "[lmn].[def].[ijk]";

           // string pattern = @"((\[lmn\]).)?((\[def\]).)?(\[ijk\])? ="; //if any number of parts is accepted on left


          string  pattern = @"(\[def\]\.\[ijk\])(\s)+="; //if only two parts are accepted

            string expected = Regex.IsMatch(mainstring,pattern)? Regex.Replace(mainstring, pattern, match => leftFillWith + " ="):mainstring;

edited Aug 2, 2017 at 7:28

answered Aug 2, 2017 at 6:43

Tommy

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9 Comments

Arc Over a year ago

can you add little explanation what is happening? is i am little confused in last two lines of your snippet

Tommy Over a year ago

pattern searches on left side of mainstring for the bracketed units. You can see there are questionmarks in the pattern, so it means that they are optional: not requiered as there may be not constant amount of unit on left side. So this pattern finds the left-sided combination and replaces it with leftFillWith by adding "=" sign at the end with " "(space) off course.

Tommy Over a year ago

@Arc your pattern was [lmn]. It searches for "l", "m", "n", but not for "lmn". These brackets are for classes ranges, but not for units. I guess you confused just because there lots of parentheses. But it helps as "combines".

Arc Over a year ago

But how can i solve my issue? brackets are there in string. What i want to do is just replace only if [def].[ijk] and not if [abc].[def].[ijk] in entire string

Arc Over a year ago

Any suggestions?

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erdi yılmaz · Accepted Answer · 2017-08-02 12:17:38Z

0

Not good but work;

       string mainstring = "[def].[ijk] = [abc].[def].[ijk]";
       string pattern = "[lmn].[def].[ijk]";
        string search ="[def].[ijk]";
        string mainstring3 = "";
       string[] mainstring2 = mainstring.Split(' ');
       for (int i = 0; i < mainstring2.Length; i++)
       {
           if (mainstring2[i] == search)
           {
               mainstring2[i] = pattern;
           }
           if (i < mainstring2.Length - 1)
           {
               mainstring3 += mainstring2[i] + " ";
           }
           else
           {
               mainstring3 += mainstring2[i];
           }
       }
       mainstring = mainstring3;
    }

edited Aug 2, 2017 at 12:17

answered Aug 2, 2017 at 6:35

erdi yılmaz

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6 Comments

Arc Over a year ago

string to replace will not only [def].[lkj], so what i did is var rege = @"\b"+stringtosearch+"\b";, this results in \b[def].[ijk], and this pattern is not present in original string

Arc Over a year ago

\b[def].[ijk]\b, this is what i am getting

Mong Zhu Over a year ago

if you want to capture [ as a character you need to use @"\[... other wise regex will interpret it as any character between the parentheses

Arc Over a year ago

But how can i solve my issue? brackets are there in string. What i want to do is just replace only if [def].[ijk] and not if [abc].[def].[ijk] in entire string. Any suggestions?

Mong Zhu Over a year ago

this will not match if the expression is standing alone on the right hand side: "[abc].[def].[ijk] = [def].[ijk]"

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