Fix creation of QgsCoordinateReferenceSystem from "user:xxxx" strings

Fixes #17254

Author: nyalldawson

src/core/qgscoordinatereferencesystem.cpp

@@ -220,7 +220,7 @@ bool QgsCoordinateReferenceSystem::createFromString( const QString &definition )
   sCrsStringLock.unlock();
 
   bool result = false;
-  QRegExp reCrsId( "^(epsg|postgis|internal)\\:(\\d+)$", Qt::CaseInsensitive );
+  QRegExp reCrsId( "^(epsg|postgis|internal|user)\\:(\\d+)$", Qt::CaseInsensitive );
   if ( reCrsId.indexIn( definition ) == 0 )
   {
     QString authName = reCrsId.cap( 1 ).toLower();

tests/src/core/testqgscoordinatereferencesystem.cpp

@@ -842,6 +842,14 @@ void TestQgsCoordinateReferenceSystem::saveAsUserCrs()
   QCOMPARE( userCrs2.srsid(), userCrs.srsid() );
   QCOMPARE( userCrs2.authid(), QStringLiteral( "USER:100000" ) );
   QCOMPARE( userCrs2.description(), QStringLiteral( "babies first projection" ) );
+
+  // createFromString with user crs
+  QgsCoordinateReferenceSystem userCrs3;
+  userCrs3.createFromString( QStringLiteral( "USER:100000" ) );
+  QVERIFY( userCrs3.isValid() );
+  QCOMPARE( userCrs3.authid(), QString( "USER:100000" ) );
+  QCOMPARE( userCrs3.toProj4(), madeUpProjection );
+  QCOMPARE( userCrs3.description(), QStringLiteral( "babies first projection" ) );
 }
 
 void TestQgsCoordinateReferenceSystem::projectWithCustomCrs()