Ruby正则表达式的优先级？

Question

I am reviewing regular expressions and cannot understand why a regular expression won't match a given string, specifically:

regex = /(ab*)+(bc)?/ 
mystring  = "abbc"

The match matches "abb" but leaves the c off. I tested this using Rubular and in IRB and don't understand why the regex doesn't match the entire string. I thought that (ab*)+ would match "ab" and then (bc)? would match "bc" .

Am I missing something in terms of precedence for regular expression operations?

Answer 1

Regular expressions try to match the first part of the regular expression as much as possible by default, and they do not backtrack to try to make larger sections match if they don't have to. Since you make (bc) optional, the (ab*) can match as much as it wants (the non-zero repetition after it doesn't have much to do) and doesn't try backtracking to try other matching alternatives.

If you want the whole string to be matched (which will force some backtracking in this case) make sure you anchor both ends of the string:

regex = /^(ab*)+(bc)?$/ 

Answer 2

The regex with parenthesis assumes you have two matches in your string.

The first one is abb because (ab*) means a and zero or more b . You have two b , so the match is abb . Then you have only c in your string, so it doesn't match the second condition which is bc .