R语言正则需求：移除单词末尾的s，但排除前导为i或s的情况

Hey there! I see you've been working with R for a while and are looking to level up your regex skills—this is a perfect practical problem to do just that. Your current sub("s$","",dfx) works for stripping any trailing 's', but we need to add a condition to skip cases where the 's' comes right after an 'i' or 's'.

The Fix: Use a Negative Lookbehind Assertion #

We can leverage a negative lookbehind in regex to tell R: "only remove the trailing 's' if it's NOT immediately preceded by 'i' or 's'". Here's the adjusted code:

dfx <- c("class","guests","trips","sassy","basis","fruits")
dfx_fixed <- sub("(?<![is])s$", "", dfx, perl = TRUE)
dfx_fixed

Let's Break Down the Regex #

• (?<![is]): This is the negative lookbehind. It checks that the character right before the trailing 's' is not 'i' or 's'. The [is] is a character class that matches either of those two letters.
• s$: Matches a literal 's' at the very end of the string (the $ denotes the end of the string).
• perl = TRUE: R's base regex functions require this flag to enable lookbehind assertions, which aren't supported in basic regex mode.

Test the Result #

When you run the code above, you'll get this output:

[1] "class"  "guest"  "trip"   "sassy"  "basis"  "fruit"

Let's verify each case matches your requirements:

• "class": Trailing 's' is preceded by 's' → remains unchanged
• "guests": Trailing 's' is preceded by 't' → stripped to "guest"
• "trips": Trailing 's' is preceded by 'p' → stripped to "trip"
• "sassy": Doesn't end with 's' → remains unchanged
• "basis": Trailing 's' is preceded by 'i' → remains unchanged
• "fruits": Trailing 's' is preceded by 't' → stripped to "fruit"

Bonus: Handling Multiple Trailing 's' #

If you ever need to strip all trailing 's' (not just the final one) except when the preceding character is 'i' or 's', you can adjust the regex to (?<![is])s+$—but this is probably beyond your current need.

Hope this helps you nail this problem and build your regex confidence!

内容的提问来源于stack exchange，提问作者user1185787