R中的gsub和正则表达式问题

Question

我在 R 中使用 gsub 将文本添加到字符串的中间。它运行良好，但由于某种原因，当位置过长时会引发错误。代码如下：

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 {}'

当括号中的数字（在本例中为 273）较少时，此代码可以正常工作，但当它很大时则不行。

---

这会产生错误：

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 默认使用 TRE 正则表达式库。限制量词中的边界从 0 到 TRE 代码中定义的 RE_DUP_MAX 有效。见this TRE reference：

绑定是以下之一，其中n 和m 是0 和RE_DUP_MAX 之间的无符号十进制整数

RE_DUP_MAX 似乎设置为 255（请参阅此 TRE source file 显示 #define RE_DUP_MAX 255），因此，您不能在 {n,m} 限制量词中使用更多。

解决方案

使用 PCRE 正则表达式风格，添加 perl = TRUE 即可。

R demo:

> 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"