Delphi TRegEx.IsMatch: How to match an empty string?-CodePudding

How to match for an empty string using Delphi (10.3) TRegEx.IsMatch?

The following cases return false where I would expect to return true:

fDoesMatch := TRegEx.IsMatch('', '^$');
fDoesMatch := TRegEx.IsMatch('', '^.{0}$');
fDoesMatch := TRegEx.IsMatch('', '\A\z');

As I've seen in the docs https://docwiki.embarcadero.com/Libraries/Sydney/en/System.RegularExpressions.TRegEx Delphi uses C library "PCRE" under the hood to do the regex.

Trying that regex patterns at https://regex101.com/ matches so i wonder why it does not work with Delphi

CodePudding user response：

TRegEx by default does not match empty strings, because it is created with undocumented roNotEmpty option.

You must explicitly exclude roNotEmpty option to enable empty string matches:

fDoesMatch := TRegEx.IsMatch('', '^$', []);

The decision to use [roNotEmpty] as the default was made to maintain backwards compatibility with older Delphi versions. See A Subtle Delphi TRegEx Change.