如何在非结构化数据中提取特定字符串之前的日期？

Question

我有一个非结构化文本，里面有很多日期，我想在单词"Message"之前提取日期。我拥有的数据如下：

21 March 2017 23:10:45 text1
21 March 2017 23:10:45  More text…..
21 March 2017 23:10:45 And more text …..
21 March 2017 23:10:45 some more text **Message:** more text 
22 March 2017 23:10:45 text1
22 March 2017 23:10:45  More text…..
22 March 2017 23:10:45 And more text …..
22 March 2017 23:10:45 some more text **Message:** more text 
23 March 2017 23:10:45 text1
23 March 2017 23:10:45  More text…..
23 March 2017 23:10:45 And more text …..
23 March 2017 23:10:45 some more text **Message:** more text 
24 March 2017 23:10:45 text1
24 March 2017 23:10:45  More text…..
24 March 2017 23:10:45 And more text …..
24 March 2017 23:10:45 some more text **Message:** more text 

并且输出将是一个新的数据格式，其中有一列表示日期：

21 March 2017 
22 March 2017 
23 March 2017 
24 March 2017

Answer 1

怎么样

sub("(?<=\\d{4}).*", "", grep("Message", txt, value=TRUE), perl=TRUE)
# [1] "21 March 2017" "22 March 2017" "23 March 2017" "24 March 2017"

我们首先使用grep()将txt还原为仅包含"Message“的值，然后使用sub()删除第一次出现四位数字后的所有文本。

数据：

txt <- readLines(textConnection("21 March 2017 23:10:45 text1
21 March 2017 23:10:45  More text…..
21 March 2017 23:10:45 And more text …..
21 March 2017 23:10:45 some more text **Message:** more text 
22 March 2017 23:10:45 text1
22 March 2017 23:10:45  More text…..
22 March 2017 23:10:45 And more text …..
22 March 2017 23:10:45 some more text **Message:** more text 
23 March 2017 23:10:45 text1
23 March 2017 23:10:45  More text…..
23 March 2017 23:10:45 And more text …..
23 March 2017 23:10:45 some more text **Message:** more text 
24 March 2017 23:10:45 text1
24 March 2017 23:10:45  More text…..
24 March 2017 23:10:45 And more text …..
24 March 2017 23:10:45 some more text **Message:** more text 
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