我建议你分几步来做：
1.提取内容中有趣的部分，它似乎在U/M和Total之间，使用像@"\r?\nU/M\r?\n(?<data>.*)\r?\nTotal\s[\d.]+"s这样的正则表达式。可能需要更改它以处理多页输出。这里我只看到 * 第1页的1*。
在这里测试：https://regex101.com/r/nf0N7z/1
1.然后，对于第二部分，我们可以看到每个项目总是以行，数量，项目单位，价格和金额开始。唯一有一点变化的是项目名称、到期日和请求者的顺序。因此，为了更清楚，在正则表达式中添加一些带有x标志的注解。我还将使用m标志，以便^和$分别匹配行的开始或结尾。
模式：

@"# Match a line such as 1 2.00 2814-212-D003 0.00 0.00
^
(?<line>\d+)\s+
(?<quantity>[\d.]+)\s+
(?<item_unit>\d+-\d+-D\d+)\s+ # Will it always be D*** ? If not then change this.
(?<price>[\d.]+)\s+
(?<amount>[\d.]+)\s+

# Match the item name (before or after EA)
(?:
  EA\s(?<item_name_v1>[^\r\n]*)\s+
  |
  (?<item_name_v2>[^\r\n]*)\nEA\s+
)

# Match the drawing
Drawing:\s+(?<drawing>[^\r\n]+)\s+

# Match the due date and requester, in several ways
Due:\s+
(?:
  Requester:\s+(?<req_date_v1>\d{2}/\d{2}/\d{4})\s+(?<req_name_v1>[^\r\n]+)
  |
  (?<req_date_v2>\d{2}/\d{2}/\d{4})\sRequester:\s+(?<req_name_v2>[^\r\n]+)
)
\s+
Order:\s+(?<order>\w+-\d+-\d+)\s+
Seq\sNo:\s+(?<seq_no>\d+)
$
"gxm

在这里测试：https://regex101.com/r/hEjKt6/2
由于有多个语法（我指的是元素的不同顺序），因此对于相同的信息，将有多个捕获组。为了清晰起见，我命名了这些组（而不仅仅是数字索引）。你可以只测试它们的内容，然后选择不为空的一个，就像下面的JavaScript例子：

// In JavaScript, the x flag isn't handled, so you end up with a messy long unreadable regex like this:
const regex = /^(?<line>\d+)\s+(?<quantity>[\d.]+)\s+(?<item_unit>\d+-\d+-D\d+)\s+(?<price>[\d.]+)\s+(?<amount>[\d.]+)\s+(?:EA\s(?<item_name_v1>[^\r\n]*)\s+|(?<item_name_v2>[^\r\n]*)\nEA\s+)Drawing:\s+(?<drawing>[^\r\n]+)\s+Due:\s+(?:Requester:\s+(?<req_date_v1>\d{2}\/\d{2}\/\d{4})\s+(?<req_name_v1>[^\r\n]+)|(?<req_date_v2>\d{2}\/\d{2}\/\d{4})\sRequester:\s+(?<req_name_v2>[^\r\n]+))\s+Order:\s+(?<order>\w+-\d+-\d+)\s+Seq\sNo:\s+(?<seq_no>\d+)$/gm;

const input = `Purchase Order
37224
(omitted details)
Total Purchase Order 0.00
PO Terms
Freight Terms:
Ship Via: FOB:
Terms:
N/A
Net 30 Days Best Way Destination
Line Quantity Item Unit Price Amount
U/M
1 2.00 2814-212-D003 0.00 0.00
EA LONG JACK PAD
Drawing: OPT
Due: Requester:
05/19/2023 NMB
Order: 2843HR-213-703 Seq No: 9002
2 2.00 2814-212-D003 0.00 0.00
EA LONG JACK PAD
Drawing: OPT
Due: 05/19/2023 Requester: NMB
Order: 2843HR-214-703 Seq No: 9002
3 2.00 2814-212-D004 0.00 0.00
SHORT JACK PAD
EA
Drawing: OPT
Due: 05/19/2023 Requester: NMB
Order: 2843HR-213-703 Seq No: 9003
4 2.00 2814-212-D004 0.00 0.00
EA SHORT JACK PAD
Drawing: OPT
Due: 05/19/2023 Requester: NMB
Order: 2843HR-214-703 Seq No: 9003
Total 0.00
(omitted details)
Page 1 of 1`;

// Loop over all matches.
for (const match of input.matchAll(regex)) {
  // An object to store all the captured groups.
  let foundData = {};
  // Loop over all captured groups.
  for (const group of Object.entries(match.groups)) {
    let name = group[0];
    let value = group[1];
    // A regex to test if it's a group with a _v1 or _v2 at the end.
    let matchMultiNamedGroup = name.match(/^(.*?)_(v\d)$/)
    if (matchMultiNamedGroup) {
      // If it's _v2, we've already handled it in the step before for _v1.
      if (matchMultiNamedGroup[2] === "v2") continue;
      // Remove the _v1 for the capturing group name.
      name = matchMultiNamedGroup[1];
      // Take the value if it's not undefined or take the capturing group _v2 instead.
      value = value !== undefined ? value : match.groups[matchMultiNamedGroup[1] + '_v2'];
    }
    // Add the group to the data object.
    foundData[name] = value;
  }
  console.log(foundData);
}

但好消息是，C#似乎接受在模式中多次使用命名捕获组。在这种情况下，这有助于我们：https://dotnetfiddle.net/M8zHBj