用R中的初始@替换带有相同单词的单词

Question

I am trying to do a dataframe string substitution in R. I need to find all the words preceded by '@' (without space, eg @word) and change the '@' for '!' (eg from @word to !word). At the same time, it leaves intact the other instances of '@' (eg @ or @@ or @[@]). For example, this is my original dataframe (to change: @def, @jkl, @stu):

> df = data.frame(number = 1:4, text = c('abc @def ghi', '@jkl @ mno', '@[@] pqr @stu', 'vwx @@@ yz'))
> df
  number          text
1      1  abc @def ghi
2      2    @jkl @ mno
3      3 @[@] pqr @stu
4      4    vwx @@@ yz

And this is what I need it to look like:

> df_result = data.frame(number = 1:4, text = c('abc !def ghi', '!jkl @ mno', '@[@] pqr !stu', 'vwx @@@ yz'))
> df_result
  number          text
1      1  abc !def ghi
2      2    !jkl @ mno
3      3 @[@] pqr !stu
4      4    vwx @@@ yz

I have tried with

> gsub('@.+[a-z] ', '!', df$text)
[1] "abc !ghi"   "!@ mno"     "!@stu"      "vwx @@@ yz"

But the result is not the desired one. Any help is much appreciated.

Thank you.

Answer 1

How about

gsub("(^| )@(\\w)", "\\1!\\2", df$text)
# [1] "abc !def ghi"  "!jkl @ mno"    "@[@] pqr !stu" "vwx @@@ yz"  

This matches an @ symbol at beginning of a string, or after a space. Then, we capture the word character after the @ symbol, and replace @ with ! .

Explanation courtesy of regex101.com :

• (^| ) is the 1st Capturing Group; ^ asserts position at start of the string; | denotes "or"; blank space matches the space character literally
• @ matches the character @ literally (case sensitive)
• (\\\\w) is the 2nd Capturing Group, it denotes a word character

The replacement string \\\\1!\\\\2 replaces the regular expression match with the first capturing group ( \\\\1 ), followed by ! , followed by the second capturing group ( \\\\2 ).

Answer 2

You can use a positive lookahead (?=...)

gsub("@(?=[A-Za-z])", "!", df$text, perl = TRUE)
[1] "abc !def ghi"  "!jkl @ mno"    "@[@] pqr !stu" "vwx @@@ yz"  

From the "Regular Expressions as used in R" documentation page:

Patterns (?=...) and (?!...) are zero-width positive and negative lookahead assertions: they match if an attempt to match the ... forward from the current position would succeed (or not), but use up no characters in the string being processed.