Match string, but only if not preceded by other string

You can use a negative lookbehind:

grep("(?<!C)lo", v, ignore.case=T, perl=T)

That will make sure that the string isn't preceded by C.


Negative Lookbehind (PCRE in R)

R uses the PCRE engine, which supports lookbehind. Do this:

grep("(?<!c)lo", subject, perl=TRUE, value=TRUE, ignore.case=TRUE);

The negative lookbehind (?<!c) asserts that what precedes the current position is not a c

Option 2: Check for Capital Letter, Turn On Case-Insensitivity Inline

Given your input, a more general option would be to assert that lo is not preceded by a capital letter:

grep("(?<![A-Z])(?i)lo", subject, perl=TRUE, value=TRUE);

For this option, we use the inline modifier (?i) to turn on case-insensitivity, but only after we have checked that no capital letters precede our position.

Reference

  • Inline Modifiers
  • Mastering Lookahead and Lookbehind
  • Lookahead and Lookbehind Zero-Length Assertions

Tags:

Regex

R

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