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I was looking though some regex documentation and was confused by something. The (R) condition in the context of (?(R)...|...) is said to be:
perl was a little cryptic:
(R)
Checks if the expression has been evaluated inside of recursion. Full syntax: (?(R)then|else)
PCRE wasn't much use:
(?(R) overall recursion condition
and regular-expressions.info had nothing to say about it.
Is this condition to say if the subroutine stack is more than 1 level deep or does it mean something else?
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If-Then-Else Conditionals in Regular Expressions. A special construct (? ifthen|else) allows you to create conditional regular expressions. If the if part evaluates to true, then the regex engine will attempt to match the then part.
Regular expressions are useful in search and replace operations. The typical use case is to look for a sub-string that matches a pattern and replace it with something else. Most APIs using regular expressions allow you to reference capture groups from the search pattern in the replacement string.
look ahead and look behind You may want to explore them to up your regular expressions game. a regular expression is a special text for identifying a pattern it can be used to search, replace, validate and extract strings matching a given pattern
If your input is more complexe, regex should be adapted, as it may not be enough restrictive (currently, it is anything between first : and last ,) Yes the regex must be adapted to your input. Assuming their is no , in the text you want to extract, you can replace .* with [^,]*. str_extract (Vec, " (?<=:) [^,]* (?=,)") is better ?
The if statement takes a condition; if the condition evaluates to TRUE, the R code associated with the if statement is executed. The condition to check appears inside parentheses, while the R code that has to be executed if the condition is TRUE, follows in curly brackets ( expr ). Suppose we have a variable x equal to -3.
back references look ahead and look behind You may want to explore them to up your regular expressions game. a regular expression is a special text for identifying a pattern it can be used to search, replace, validate and extract strings matching a given pattern
See this explanation:
if there is no subpattern named 'R', the condition is true if a recursive call to the whole pattern or any subpattern has been made
This implies that (?(R) condition checks if the whole pattern was recursed at least once, and the result of the check is boolean, either True if recursion took place, or False otherwise.
If you need to check some examples, see https://github.com/PhilipHazel/pcre2/blob/587b94277b50ababde2380b5877c93e36ca65db8/src/pcre2_jit_test.c.
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