JavaScript Alternative To Possessive Quantifiers In Dynamic Regex
I'm using JavaScript to extract a subset of "siblings" from a comma-delimited string of members I call a "generation" string.
Metaphorically speaking, the members are all from the same generation, but they are not all siblings (from the same parents). Here's an example:
// This is the generation string to search
var generation = 'ABAA,ABAB,ABAC,ABAD,ABBA,ACAA,ACAB,ACAD,AEAB,AEAD,AFAA';
// This is the member for whom to extract siblings (member included)
var member = 'ACAA';
The generation string and its members have the following characteristics:
Continuing the example...
// This is how I go about extracting the desired result: ACAA,ACAB,ACAD
var mParent = member.substr(0, member.length - 1) ;
var mPattern = mParent + '[A-Z]';
var mPattern = '(.*)((' + mPattern + ')(,$1)*)(.*)'; // Trouble is here
var mRegex = new RegExp(mPattern);
var mSiblings = generation.replace(mRegex, '$2');
The trouble spot identified above concerns regex quantifiers in the constructed pattern. As it is above, everything is set to greedy, so the value of mSiblings is:
ACAD
That's only the last member. Changing mPattern to be less greedy in hopes of extracting the other members yields the following
// Reluctant first expression yields ACAA
var mPattern = '(.*?)((' + mPattern + ')(,$1)*)(.*)';
// Reluctant last expression yields ACAD,AEAB,AEAD,AFAA
var mPattern = '(.*)((' + mPattern + ')(,$1)*)(.*?)';
// Reluctant first and last yields ACAA,ACAB,ACAD,AEAB,AEAD,AFAA
var mPattern = '(.*?)((' + mPattern + ')(,$1)*)(.*?)';
If I could make the middle expression possessive, this would be problem solved. Something like this:
// Make as many "middle" matches as possible by changing (,$1)* to (,$1)*+
var mPattern = '(.*?)((' + mPattern + ')(,$1)*+)(.*?)';
But as I have read (and have the syntax errors to prove it), JavaScript doesn't support possessive regular expression quantifiers. Can someone suggest a solution? Thank you.
The most obvious problem is the $1 . Within a regex, you would refer to capturing group #1 using 1 , not $1 . The (,$1)* in your regex is never going to match anything. But a group reference isn't going to do any good anyway.
When you use a group reference in a regex, you aren't applying that part of the regex again, you're simply matching the same thing that it matched the first time. That is, (ACA[AZ])(,1)* will match ACAA,ACAA , but not ACAA,ACAB or ACAA,ACAC . If you want to do that, you need to repeat the actual regex: (ACA[AZ])(,ACA[AZ])* . Since you're generating the regex dynamically, that shouldn't be a problem.
Note that that's the whole regex: ACA[AZ](,ACA[AZ])* . There's no need to match the stuff preceding or following the part that interests you; that's just making the job more complicated (and the results more confusing). You can access the match result directly, instead of using that "replace" gimmick:
var match = mRegex.exec(generation);
if (match != null) {
mSiblings = match[0];
}
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