R 中的捕获组

Question

I have then following pattern Set(?:Value)? in R as follows:

grepl('Set(?:Value)?', 'Set(Value)', perl=T)

this pattern is macthed by

1- Set
2- Set Value
3- Set(Value)

But I want to match only for two first cases and for for third case. Can anybody help me?

Thank you

Answer 1

You can use

grepl('^Set(?:\\s+Value)?$', x)
grepl('\\bSet(?!\\(Value\\))(?:\\s+Value)?\\b', x, perl=TRUE)

See regex demo #1 and regex demo #2 .

Details :

• ^Set(?:\\s+Value)?$ - start of string, Set , an optional sequence of one or more whitespaces ( \s+ ) and a Value and then end of string
• \bSet(??\(Value\))(:?\s+Value)?\b :
  • \b - word boundary
  • Set - Set string
  • (?!\(Value\)) - no (Value) string allowed at this very location
  • (?:\s+Value)? - an optional sequence of one or more whitespaces ( \s+ ) and a Value
  • \b - word boundary

See an R demo :

x <- c("Set", "Set Value", "Set(Value)")
grep('^Set(?:\\s+Value)?$', x, value=TRUE)
## => [1] "Set"       "Set Value"
grep('\\bSet(?!\\(Value\\))(?:\\s+Value)?\\b', x, perl=TRUE, value=TRUE)
## => [1] "Set"       "Set Value"