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Regex: "(^ |)" vs "(| ^)"

I have a special question about regular expressions in R:

grepl("(|^)over","stackoverflow") # [1] TRUE grepl("(^|)over","stackoverflow") # [1] FALSE grepl("(^|x|)over","stackoverflow") # [1] FALSE grepl("(x|^|)over","stackoverflow") # [1] FALSE grepl("(x||^)over","stackoverflow") # [1] TRUE

Why aren't all of these expressions evaluated before TRUE ?

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POSIX regular expressions should actually make all of these True. It appears that R is using a slightly modified version of the Ville Laurikari TRE library , which is not quite up to standard. I will follow @rawr's guidelines and use perl = TRUE for more compatible regular expressions.

See also: If both halves of an OR regular expression group match, will it be selected?

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