[SOLVED] grep in R, literal and pattern match

Issue

I have seen in manuals how to use grep to match either a pattern or an exact string. However, I cannot figure out how to do both at the same time. I have a latex file where I want to find the following pattern:

\caption[SOME WORDS]

and replace it with:

\caption[\textit{SOME WORDS}]

I have tried with:

texfile <- sub('\\caption[','\\caption[\\textit ', texfile, fixed=TRUE)

but I do not know how to tell grep that there should be some text after the square bracket, and then a closed square bracket.

Solution

You can use

texfile <- "\\caption[SOME WORDS]" ## -> \caption[\textit{SOME WORDS}]
texfile <-gsub('(\\\\caption\\[)([^][]*)]','\\1\\\\textit{\\2}]', texfile)
cat(texfile)
## -> \caption[\textit{SOME WORDS}]

See the R demo online.

Details:

• (\\caption\[) - Group 1 (\1 in the replacement pattern): a \caption[ string
• ([^][]*) - Group 2 (\2 in the replacement pattern): any zero or more chars other than [ and ]
• ] - a ] char.

Another solution based on a PCRE regex:

gsub('\\Q\\caption[\\E\\K([^][]*)]','\\\\textit{\\1}]', texfile, perl=TRUE)

See this R demo online. Details:

• \Q - start "quoting", i.e. treating the patterns to the right as literal text
• \caption[ - a literal fixed string
• \E - stop quoting the pattern
• \K - omit text matched so far
• ([^][]*) - Group 1 (\1): any zero or more non-bracket chars
• ] - a ] char.

Answered By - Wiktor Stribiżew

Answer Checked By - Senaida (WPSolving Volunteer)