How to convert a vector of strings to Title Case

The main problem is that you're missing perl=TRUE (and your regex is slightly wrong, although that may be a result of flailing around to try to fix the first problem).

Using [:lower:] instead of [a-z] is slightly safer in case your code ends up being run in some weird (sorry, Estonians) locale where z is not the last letter of the alphabet ...

re_from <- "\\b([[:lower:]])([[:lower:]]+)"
strings <- c("first phrase", "another phrase to convert",
             "and here's another one", "last-one")
gsub(re_from, "\\U\\1\\L\\2" ,strings, perl=TRUE)
## [1] "First Phrase"              "Another Phrase To Convert"
## [3] "And Here's Another One"    "Last-One"    

You may prefer to use \\E (stop capitalization) rather than \\L (start lowercase), depending on what rules you want to follow, e.g.:

string2 <- "using AIC for model selection"
gsub(re_from, "\\U\\1\\E\\2" ,string2, perl=TRUE)
## [1] "Using AIC For Model Selection"

Without using regex, the help page for tolower has two example functions that will do this.

The more robust version is

capwords <- function(s, strict = FALSE) {
    cap <- function(s) paste(toupper(substring(s, 1, 1)),
                  {s <- substring(s, 2); if(strict) tolower(s) else s},
                             sep = "", collapse = " " )
    sapply(strsplit(s, split = " "), cap, USE.NAMES = !is.null(names(s)))
}
capwords(c("using AIC for model selection"))
## ->  [1] "Using AIC For Model Selection"

To get your regex approach (almost) working you need to set `perl = TRUE)

gsub("(\\b[a-z]{1})", "\\U\\1" ,strings, perl=TRUE)


[1] "First Phrase"              "Another Phrase To Convert"
[3] "And Here'S Another One"    "Last-One"  

but you will need to deal with apostrophes slightly better perhaps

sapply(lapply(strsplit(strings, ' '), gsub, pattern = '^([[:alnum:]]{1})', replace = '\\U\\1', perl = TRUE), paste,collapse = ' ')

A quick search of SO found https://stackoverflow.com/a/6365349/1385941

Already excellent answers here. Here's one using a convenience function from the reports package:

strings <- c("first phrase", "another phrase to convert",
    "and here's another one", "last-one")

CA(strings)

## > CA(strings)
## [1] "First Phrase"              "Another Phrase To Convert"
## [3] "And Here's Another One"    "Last-one"       

Though it doesn't capitalize one as it didn't make sense to do so for my purposes.

Update I manage the qdapRegex package that has the TC (title case) function that does true title case:

TC(strings)

## [[1]]
## [1] "First Phrase"
## 
## [[2]]
## [1] "Another Phrase to Convert"
## 
## [[3]]
## [1] "And Here's Another One"
## 
## [[4]]
## [1] "Last-One"