返回较大字符串中特定字符串之前的时间正则表达式的第一个出现

Question

我已经在线阅读了如何在字符串之后找到字符串的第一个匹配项，但是我试图在字符串之前找到它。

我最近才开始使用正则表达式，所以这可能是一个非常简单的问题。

示例文本：

03:47:06 This is line 1
03:47:07 This is line 2
03:47:08 This is line 3
03:47:09 This is line 4
This is line 5
03:47:10 This is line 6
03:47:11 This is line 7
This is line 8

我希望能够使变量返回特定字符串之前的第一个时间戳。

例如，第4行的时间戳，我需要一个变量以返回03:47:09

我有正则表达式：

/\d\d:\d\d:\d\d/

我大概可以通过遍历寻找正则表达式的每一行来解决该问题，直到出现文本“第4行”，但是由于我认为会有很多行，因此必须有一种更简单的方法？

Answer 1

其他答案很好，但是如果您只需要在开始时进行匹配，则使用此方法：

var string_of_text = `03:47:06 This is line 1
03:47:07 This is line 2
03:47:08 This is line 3
03:47:09 This is line 4
This is line 5
03:47:10 This is line 6
03:47:11 This is line 7
This is line 8 03:33:12`;
var start_timestamp = string_of_text.match(/^([\:\d]){8}/gm)

["03:47:06", "03:47:07", "03:47:08", "03:47:09", "03:47:10", "03:47:11"]

就是说，根据您的评论，我猜您正在尝试将返回的时间与行相匹配...？在这种情况下，您应该先分割行：

string_of_text.split('\n').map(v=>(v.match(/^([\:\d]){8}/g)||[''])[0]);

0: "03:47:06"
1: "03:47:07"
2: "03:47:08"
3: "03:47:09"
4: ""
5: "03:47:10"
6: "03:47:11"
7: ""

如果您需要在任何地方进行匹配，只需从正则表达式中删除^：

string_of_text.split('\n').map(v=>(v.match(/([\:\d]){8}/g)||[''])[0]);

0: "03:47:06"
1: "03:47:07"
2: "03:47:08"
3: "03:47:09"
4: ""
5: "03:47:10"
6: "03:47:11"
7: "03:33:12"

说明：

string_of_text
  .split('\n') // Split into an array of lines.
  .map( // Replace each entry with a new one using the given function.
     v => ( v.match(/([\:\d]){8}/g) || [''] )[0] // Each array item (v) is searched for the pattern.
                                                 // If not found, null is returned, so default to an array with an empty string.
   );

Answer 2

这个解决了您的问题，仅选择此特定模式：

/([\:\d]){8}/g

请参见下面的链接中的生活代码：

Live example

Answer 3

使用正则表达式：

([\d+|:]+) This is line 4

https://regex101.com/r/39qC3F/2

仅使用“第4行”进行编辑：

([\d+|:]+)[\w ]*line 4[\w ]*

https://regex101.com/r/39qC3F/3

使用示例：

const pattern = 'line 4';
const regex = new RegExp(`([\\d+|:]+)[\\w ]*${pattern}[\\w ]*`, 'gm');
const str = `03:47:06 This is line 1
03:47:07 This is line 2
03:47:08 This is line 3
03:47:09 This is line 4
This is line 5
03:47:10 This is line 6
03:47:11 This is line 7
This is line 8`;
let m;

while ((m = regex.exec(str)) !== null) {
    // This is necessary to avoid infinite loops with zero-width matches
    if (m.index === regex.lastIndex) {
        regex.lastIndex++;
    }
    
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}