regex_math_op01

#ME:
x <- "x + b + (c2 -(d3 + wo22^0.2/3/123))"
strsplit(gsub("\\(|\\)|\\s|[^a-zA-Z0-9](?:[+-\\*\\^\\/]{1})+[0-9.]","",x),"[^0-9a-zA-Z]")
#[[1]]
#[1] "x"    "b"    "c2"   "d3"   "wo22"

# https://www.ptt.cc/bbs/R_Language/M.1544673627.A.D09.html from celestialgod
all.vars(as.formula(paste0(x, "~1")))
#[1] "x"    "b"    "c2"   "d3"   "wo22"