How to get group matches of regular expressions in CMake?

cmake, regex

Solution

CMake's regular expressions are relatively limited. Look at the `static char* regatom (int *flagp)` method in RegularExpression.cxx. A `\\` indicates that the next character is escaped (treated literally). It looks like there are no back-references possible in the CMake regex.

As a work around, you can invoke shell commands using `execute_process`.

set(TEXT "ab,cc,df,gg")
message("TEXT: ${TEXT}")

execute_process(
    COMMAND echo ${TEXT}
    COMMAND sed "s/.*\\(\\(.\\)\\2\\).*/\\1/g"
    OUTPUT_VARIABLE RESULT OUTPUT_STRIP_TRAILING_WHITESPACE
    )

message("RESULT: ${RESULT}")

This produces:

TEXT: ab,cc,df,gg
RESULT: gg

You will have to adjust your regex do produce `cc;gg` from the given string.

Problem

Let's look at the following illustrative example. ``` set(TEXT "ab,cc,df,gg") string(REGEX MATCHALL "((.)\\2)" RESULT "${TEXT}") message("Result: ${RESULT}") # Expected: Result: cc;gg # Actual: Result: ``` Compare the expected result on regex101. Does anyone know how to retrieve match group 1 correctly in the above example? Is this possible at all with CMake? I couldn't find much on the limitations of the regular expression processor used by CMake in the web. Who knows more? (There's a little something written about this in CMake FAQ) Thanks for the support!

Original source