使用R语言提取指定起止模式间字符串遇NA问题求助

问题场景 #

使用stringr包提取字符串中特定起始到结束模式间的内容，代码运行后输出NA，期望提取的内容为：
"Purchased 2, one had a quality issue but they sent a replacement\n \nReviewed in the United States"

原代码：

library(stringr)    
s1="Ben Fisher5.0 out of 5 stars\n\n\n\n\n\n\n\n \n \n Purchased 2, one had a quality issue but they sent a replacement\n \nReviewed in the United States <U+0001F1FA><U+0001F1F8>"
start_pattern <- "out of 5 stars\n\n\n\n\n\n\n\n \n \n"
end_pattern <- "&lt;U+0001F1FA&gt;"
str_extract(s1, paste0("(?<=", start_pattern, ").*(?=",end_pattern  , ")"))

问题原因 #

1. 起始模式匹配偏差：原字符串中起始模式末尾带有一个空格（out of 5 stars\n\n\n\n\n\n\n\n \n \n ），但代码里的start_pattern漏掉了这个空格，导致无法定位到正确的起始匹配位置。
2. 正则换行匹配限制：默认正则规则中，.不会匹配换行符\n，而目标内容包含换行，因此.*无法覆盖换行后的部分，最终匹配失败返回NA。

修复方案 #

方案1：修正起始模式并开启单行模式 #

library(stringr)    
s1="Ben Fisher5.0 out of 5 stars\n\n\n\n\n\n\n\n \n \n Purchased 2, one had a quality issue but they sent a replacement\n \nReviewed in the United States <U+0001F1FA><U+0001F1F8>"
# 修正起始模式，补上末尾的空格
start_pattern <- "out of 5 stars\n\n\n\n\n\n\n\n \n \n "
end_pattern <- "&lt;U+0001F1FA&gt;"
# 用(?s)开启单行模式，让.匹配所有字符（包括换行）
result <- str_extract(s1, paste0("(?s)(?<=", start_pattern, ").*(?=", end_pattern, ")"))
# 可选：去除首尾多余空格
result <- str_trim(result)
print(result)

方案2：用[\s\S]*代替.*匹配所有字符 #

如果不想开启单行模式，可使用[\s\S]*匹配所有空白与非空白字符（包含换行）：

library(stringr)    
s1="Ben Fisher5.0 out of 5 stars\n\n\n\n\n\n\n\n \n \n Purchased 2, one had a quality issue but they sent a replacement\n \nReviewed in the United States <U+0001F1FA><U+0001F1F8>"
start_pattern <- "out of 5 stars\n\n\n\n\n\n\n\n \n \n "
end_pattern <- "&lt;U+0001F1FA&gt;"
result <- str_extract(s1, paste0("(?<=", start_pattern, ")[\\s\\S]*(?=", end_pattern, ")"))
result <- str_trim(result)
print(result)

两种方案均可输出期望内容：
"Purchased 2, one had a quality issue but they sent a replacement\n \nReviewed in the United States"

内容的提问来源于stack exchange，提问作者student_R123