[R] discourage use of regex for fixed string comparisons (#8736)

demo/kaggle-otto/otto_train_pred.R

@@ -7,7 +7,7 @@ train <- train[, -1]
 test <- test[, -1]
 
 y <- train[, ncol(train)]
-y <- gsub('Class_', '', y)
+y <- gsub('Class_', '', y, fixed = TRUE)
 y <- as.integer(y) - 1  # xgboost take features in [0,numOfClass)
 
 x <- rbind(train[, -ncol(train)], test)

demo/kaggle-otto/understandingXGBoostModel.Rmd

@@ -87,7 +87,7 @@ For that purpose, we will:
 ```{r classToIntegers}
 # Convert from classes to numbers
 y <- train[, nameLastCol, with = FALSE][[1]] %>%
-    gsub('Class_', '', .) %>%
+    gsub('Class_', '', ., fixed = TRUE) %>%
     as.integer %>%
     subtract(., 1)