What regular expression engine does QGIS use?

Question

Where can I find the documentation about QGIS regular expression engine and how to craft a pattern with it. I see the following regex functions documented

• regexp_replace
• regexp_substr
• regexp_matches
• regexp_match

Answer 1

QGIS, uses QT's QRegularExpression. which is itself just PCRE.

QRegularExpression implements Perl-compatible regular expressions. It fully supports Unicode. For an overview of the regular expression syntax supported by QRegularExpression, please refer to the aforementioned pcrepattern(3) man page. A regular expression is made up of two things: a pattern string and a set of pattern options that change the meaning of the pattern string.

You can see the use of it in src/core/qgsexpression.cpp

Note one Gotcha with the QT C++ implementation is that you have to escape \,

Note that due to C++ literal strings rules, you must escape all backslashes inside the pattern string with another backslash:

// matches two digits followed by a space and a word
QRegularExpression re("\\d\\d \\w+");

// matches a backslash
QRegularExpression re2("\\\\");

Another but very minor non-gotcha, is

QRegularExpression does not support all the features available in Perl-compatible regular expressions. The most notable one is the fact that duplicated names for capturing groups are not supported, and using them can lead to undefined behaviour.