Regex pattern for checking if a string starts with a certain substring?

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What's the regular expression to check if a string starts with for example ==> "RE" "4T" AND "4S"
The idea is I want to delete any line which starts with the above patter
but should any line should contain say "RE" IN-BETWEEN IT SHOULD NOT DELETE .
I just want to delete it when it begins with these=>"RE" "4T" AND "4S"
These what I have so far but it deletes lines that contains the above strings
Like if a line is CODEpROJECT_RESTING-WH It would format everything from RESTING-WH .But I would like to delete the whole line if it starts with "RE"

C#

string rchTxtContent.Text;

     //This Line of code do formats for ==>"RE" within the RichTextBox at_all_times.
     var cLearOut_RE_WithinRichTextBox = new Regex(@"RE(.*)", RegexOptions.Multiline);
     rchTxtContent.Text = cLearOut_RE_WithinRichTextBox.Replace(rchTxtContent.Text, string.Empty);
     rchTxtContent.Lines = (from code_Projects in rchTxtContent.Lines
                            let x = code_Projects.IndexOf("RE")
                            select (x >= 0 ? code_Projects.Substring(0, x) : code_Projects)).ToArray();

Thanks for any advice.

Posted 4-Jun-15 5:03am

Julia Fraveau

Updated 4-Jun-15 5:09am

PIEBALDconsult

v2

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PIEBALDconsult 4-Jun-15 11:19am

^RE(.*)

Julia Fraveau 4-Jun-15 11:27am

Ok PIEBALDconsult right away sir.

Julia Fraveau 4-Jun-15 11:29am

Just tried your solution,but is the same like mine @PIEBALDConsult.

2 solutions

Solution 2

following on from Griff's solution:

C#

Regex matcher = new Regex("^(RE|4T|4S)", RegexOptions.ExplicitCapture | RegexOptions.Compiled);
rchTxtContent.Lines = (from code_Projects in rchTxtContent.Lines
                       where !matcher.IsMatch(code_Projects)
                       select code_Projects).ToArray();

This will just remove the lines that begin with any of the 3 two-character strings.

Additional considerations:
If this is a compile-time fixed pattern, then make the matcher a private static readonly field of the class (instead of a local variable of the method) and then the construction and compilation (relatively expensive) of the Regex will happen only once.
You can add the RegexOptions.IgnoreCase to the constructor if you need this to be case insensitive.

Posted 4-Jun-15 7:38am

Matt T Heffron

Comments

Andreas Gieriet 4-Jun-15 15:59pm

My 5!
Cheers
Andi
PS: Same here as for OriginalGriff: You might use a non-capturing group (?:...) instead of the capturing group (...).

Matt T Heffron 4-Jun-15 16:36pm

But the RegexOptions.ExplicitCapture means that the unnamed group does not actually capture.

Andreas Gieriet 5-Jun-15 3:21am

You are right - I missed that option in the code...
I'm so used to have both kind of groups in regular expressions that I never use that option.
Cheers
Andi

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OriginalGriff · Accepted Answer · 2015-06-04T05:15:00

Solution 1

Try:

^(RE|4T|4S)

Posted 4-Jun-15 5:15am

OriginalGriff

Comments

Julia Fraveau 4-Jun-15 11:26am

Thanks @OriginalGiff, you mean in the place of the IndexOf?

OriginalGriff 4-Jun-15 11:50am

Your questions was:
"What's the regular expression to check if ..."
So I'd start by looking at using it in the regex myself...:laugh:

Julia Fraveau 4-Jun-15 11:56am

You are the man @OginalGiff..

Andreas Gieriet 4-Jun-15 15:59pm

My 5!
Cheers
Andi
PS: You might use a non-capturing group (?:...) instead of the capturing group (...).