Clarification about Regular Expression Limit

Question

Can someone help me understand the following statement from the Apex Developer Guide? I'm trying to understand regular expressions, but this statement has me a bit confused, and I can't figure out how/when a developer could hit this limit:

Salesforce limits the number of times an input sequence for a regular expression can be accessed to 1,000,000 times. If you reach that limit, you receive a runtime error.

How/when is a regular expression accessed? Can you give a simple use case of when something like this could happen?

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_pattern_and_matcher_using.htm

The reason I'm asking, is that we are planning to put a batch process in place to evaluate all phone fields on our Accounts and Contacts, and if the country code is US or Canada, we'll add (if it's not already there) a '+1' to the phone number so that the number can be auto-dialed from any country of origin. We have 5 phone fields on Contact alone, and we're concerned about limits.

Answer 1

I belive what it's referring to is the number of times a string is accessed while processing a regex against it.

Regular expressions can do all sorts of weird and wonderful things with back-tracking etc. that means they have to run backwards and forwards over the String many times to decide if it's a match. I think this limit is referring to the number of times the variable needs to be read during a regex evaluation, and I can only see this limit being hit with a complicated expression on a long string (think HTML page body or something).

For your use case you'll have no need for concern regarding this limit.

Answer 2

It has to do with the complexity of the Matcher. In practical terms your Matcher String can't be over 1M or you will hit that limit.

https://help.salesforce.com/apex/HTViewSolution?id=000005024&language=en_US

Your use case should be OK