R中gsub和regex的問題

Question

我在R中使用gsub將文本添加到字符串的中間。 它工作得很好，但由於某種原因，當位置太長時，它會拋出錯誤。 代碼如下：

gsub(paste0('^(.{', as.integer(loc[1])-1, '})(.+)$'), new_cols, sql)

 Error in gsub(paste0("^(.{273})(.+)$"), new_cols, sql) : invalid regular expression '^(.{273})(.+)$', reason 'Invalid contents of {}' 

當括號中的數字（在這種情況下為273）較小時，此代碼可以正常工作，但當它很大時，則不行。

---

這會產生錯誤：

sql <- "The cat with the bat went to town. He ate the fat mat and wouldn't stop til the sun came up. He was a fat cat that lived with a rat who owned many hats.The cat with the bat went to town. He ate the fat mat and wouldn't stop til the sun came up. He was a fat cat that lived with a rat who owned many hats."  
new_cols <- "happy" 
gsub('^(.{125})(.+)$', new_cols, sql)  #**Works
gsub('^(.{273})(.+)$', new_cols, sql) 

 Error in gsub("^(.{273})(.+)$", new_cols, sql) : invalid regular expression '^(.{273})(.+)$', reason 'Invalid contents of {}' 

Answer 1

背景

R gsub默認使用TRE regex庫。 限制量詞中的邊界在從TRE代碼中定義的0到RE_DUP_MAX之間有效。 看到這個TRE參考 ：

綁定是以下之一，其中n和m是0和RE_DUP_MAX之間的無符號十進制整數

似乎RE_DUP_MAX設置為255（參見顯示#define RE_DUP_MAX 255 TRE源文件 ），因此，您不能在{n,m}限制量詞中使用更多。

解

使用PCRE正則表達式風格，添加perl = TRUE ，它將起作用。

R演示 ：

> sql <- "The cat with the bat went to town. He ate the fat mat and wouldn't stop til the sun came up. He was a fat cat that lived with a rat who owned many hats.The cat with the bat went to town. He ate the fat mat and wouldn't stop til the sun came up. He was a fat cat that lived with a rat who owned many hats."
> new_cols <- "happy"
> gsub('^(.{273})(.+)$', new_cols, sql, perl=TRUE)
[1] "happy"