A flag is part of the regular-expression contract, not a display preference. RegexLab passes the exact unique flag string to the current JavaScript runtime.
g and y control repeated execution
Global searches continue after a match; sticky searches require the next match at lastIndex. Without either flag, only one exec result is collected.
u and v change Unicode parsing
They affect escapes, code-point matching and safe advancement after an empty match. Runtime support for v is detected by compilation.
d adds index evidence
The flag may enrich native match objects, while RegexLab always reports the complete match index and length plus captures.