Match string, but only if not preceded by other string

Question

Say I have vector of strings:

v = c("SPX.Close", "AAPL.Low", "Lo", "LowPrice", "PriceLow", "low")

How to write regex that would match all strings resembling phrase "low"?

grep("lo", v, ignore.case=T) # 1 2 3 4 5 6 7

This matches the first string too, which I don't want.

How to match lo only if not preceded by letter c ?

Answer 1

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

Answer 2

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.