我需要帮助来构建正则表达式

Question

It is my first time working with regex and I am a little lost. To give you a little background, I am making a program that reads a text file line by line and it saves in a string called "line". If the line starts with either a tab o or a whitespace, followed by a number or number and dots (such as 1 or 1.2.1, for instance) followed by another tab or whitespace, it copies the line to another file.

So far I build this regex, but it does not work

            string pattern = @"(\t| ) *[0-9.] (\t| )";

            if (line.StartsWith(pattern))
            {

                //copy line

            }

Also, is line.StartsWith correct? Or should I use something like rgx.Matches(pattern)?

Answer 1

Your pattern contains a character class without a quantifier, which will match either a single digit or dot.

To prevent matching for example only dots you could first match digits followed by an optional part which matches a dot and then again digits [0-9]+(?:\\.[0-9]+)*

Note that in this part (\\t| ) there are 2 characters expected to match as the space in that part has meaning.

You could simplify the pattern to use a character class to match either a tab or space instead of using an alternation and if you don't need the capturing group you could omit it.

Instead of using StartsWith you could usefor example IsMatch

^[ \t][0-9]+(?:\.[0-9]+)*[ \t]

• ^ Start of string
• [ \\t] Match a single tab or space
• [0-9]+ Match 1+ digits 0-9
• (?:\\.[0-9]+)* Repeat 0+ times a dot and 1+ digits
• [ \\t] Match a single tab or space

Regex demo | C# demo

For example

string s = "\t1.2.1 ";
Regex regex = new Regex(@"^[ \t][0-9]+(?:\.[0-9]+)*[ \t]");

if (regex.IsMatch(s)) {
    //copy line
}