r grep by regex - finding a string that contains a sub string exactly one once

Question

I am using R in Ubuntu, and trying to go over list of files, some of them i need and some of them i don\'t need,

I try to get the one\'s i need by finding a sub string

Answer 1

Detecting strings with a but not aa

You can use the following TRE regex:

^[^a]*a[^a]*$

It matches the start of the string (^), 0+ chars other than a ([^a]*), an a, again 0+ non-'a's and the end of string ($). See this IDEONE demo:

a <- c("aca","cac","a", "abab", "ab-ab", "ab-cc-ab")
grep("^[^a]*a[^a]*$", a, value=TRUE)
## => [1] "cac" "a"

Finding Whole Word Containing a but not aa

If you need to match words that have one a only, but not two or more as inside in any location.

Use this PCRE regex:

\b(?!\w*a\w*a)\w*a\w*\b

See this regex demo.

Explanation:

• \b - word boundary
• (?!\w*a\w*a) - a negative lookahead failing the match if there are 0+ word chars, a, 0+ word chars and a again right after the word boundary
• \w* - 0+ word chars
• a - an a
• \w* - 0+ word chars
• \b - trailing word boundary.

NOTE: Since \w matches letters, digits and underscores, you might want to change it to \p{L} or [^\W\d_] (only matches letters).

See this demo:

a <- c("aca","cac","a")
grep("\\b(?!\\w*a\\w*a)\\w*a\\w*\\b", a, perl=TRUE, value=TRUE)
## => [1] "cac" "a"