Question

I want 4 general RegEx expressions for the following 4 basic cases:

1. Up to A chars starting after B chars from start of line on up to C lines starting after D lines from start of file
2. Up to A chars starting after B chars from start of line on up to C lines occurring before D lines from end of file
3. Up to A chars starting before B chars from end of line on up to C lines starting after D lines from start of file
4. Up to A chars starting before B chars from end of line on up to C lines starting before D lines from end of file

These would allow to select arbitrary text blocks anywhere in the file.

So far I have managed to come up with cases that only work for lines and chars separately:

• (?<=(?m:^[^\r]{N}))[^\r]{1,M} = UP
  TO M chars OF EVERY LINE, AFTER FIRST
  N chars
• [^\r]{1,M}(?=(?m:.{N}\r$))
  = UP TO M chars OF EVERY LINE, BEFORE LAST N chars

The above 2 expressions are for chars, and they return MANY matches (one for each line).

• (?<=(\A([^\r]*\r\n){N}))(?m:\n*[^\r]*\r$){1,M} = UP TO M lines AFTER FIRST N lines
• (((?=\r?)\n[^\r]*\r)|((?=\r?)\n[^\r]+\r?)){1,M}(?=((\n[^\r]*\r)|(\n[^\r]+\r?)){N}\Z) = UP TO M lines BEFORE LAST N lines from end

These 2 expressions are equivalents for the lines, but they always return just ONE match.

The task is to combine these expressions to allow for scenarios 1-4. Anyone can help?

Note that the case in the title of the question, is just a subclass of scenario #1, where both B = 0 and D = 0.

EXAMPLE 1: Characters 3-6 of lines 3-5. A total of 3 matches.

SOURCE:

line1 blah 1
line2 blah 2
line3 blah 3
line4 blah 4
line5 blah 5
line6 blah 6

RESULT:

<match>ne3 </match>
<match>ne4 </match>
<match>ne5 </match>

EXAMPLE 2: Last 4 characters of 2 lines before 1 last line. A total of 2 matches.

SOURCE:

line1 blah 1
line2 blah 2
line3 blah 3
line4 blah 4
line5 blah 5
line6 blah 6

RESULT:

<match>ah 4</match>
<match>ah 5</match>

Answer 1

For starters, here’s an answer for “Basic Case 1”:

Regex regexObj = new Regex(
    @"(?<=            # Assert that the following can be matched before the current position
     \A               # Start of string
     (?:.*\r\n){2,4}  # 2 to 4 entire lines (D = 2, C = 4+1-2)
     .{2}             # 2 characters (B = 2)
    )                 # End of lookbehind assertion
    .{1,3}            # Match 1-3 characters (A = 3)", 
    RegexOptions.IgnorePatternWhitespace);

You can now iterate over the matches using

Match matchResults = regexObj.Match(subjectString);
while (matchResults.Success) {
    // matched text: matchResults.Value
    // match start: matchResults.Index
    // match length: matchResults.Length
    matchResults = matchResults.NextMatch();
}

So, in the text

line1 blah 1
line2 blah 2
line3 blah 3
line4 blah 4
line5 blah 5
line6 blah 6

it will match

ne3
ne4
ne5