问2个字符串之间的正则表达式值

EN

(?<=d)\d+(?=_)

(?<=d) # Assert that the previous character is a d
\d+    # Match one or more digits
(?=_)  # Assert that the following character is a _

resultString = Regex.Match(subjectString, @"(?<=d)\d+(?=_)").Value;

d([^_]+)

d       # Match d
([^_]+) # Match  (and capture) one or more characters that isn't a _

//your string
string theString = "d1048_m325";

//chars to find to cut the middle string
char firstChar = 'd';
char secondChar = '_';

//find the positions of both chars
//firstPositionOfFirstChar +1 to not include the char itself
int firstPositionOfFirstChar = theString.IndexOf(firstChar) +1; 
int firstPositionOfSecondChar = theString.IndexOf(secondChar);

//the middle string will have a length of firstPositionOfSecondChar - firstPositionOfFirstChar  
int middleStringLength = firstPositionOfSecondChar - firstPositionOfFirstChar;

//cut!
string middle = theString.Substring(firstPositionOfFirstChar, middleStringLength);