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regexrlookbehind

R: workaround for variable-width lookbehind

Given this vector:

ba <- c('baa','aba','abba','abbba','aaba','aabba')'

I want to change the final a of each word to i except baa and aba.

I wrote the following line ...

gsub('(?<=a[ab]b{1,2})a','i',ba,perl=T)

but was told: PCRE pattern compilation error 'lookbehind assertion is not fixed length' at ')a'.

I looked around a little bit and apparently R/Perl can only lookahead for a variable width, not lookbehind. Any workaround to this problem? Thanks!

Solution

  • You can use the lookbehind alternative \K instead. This escape sequence resets the starting point of the reported match and any previously consumed characters are no longer included.

    Quoted — rexegg

    The key difference between \K and a lookbehind is that in PCRE, a lookbehind does not allow you to use quantifiers: the length of what you look for must be fixed. On the other hand, \K can be dropped anywhere in a pattern, so you are free to have any quantifiers you like before \K.

    Using it in context:

    sub('a[ab]b{1,2}\\Ka', 'i', ba, perl=T)
# [1] "baa"   "aba"   "abbi"  "abbbi" "aabi"  "aabbi"

    Avoiding lookarounds:

    sub('(a[ab]b{1,2})a', '\\1i', ba)
# [1] "baa"   "aba"   "abbi"  "abbbi" "aabi"  "aabbi"