Regular Expressions 101

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/
^Chào thế giới
/
gmui
^ asserts position at start of a line
Chào thế giới
matches the characters Chào thế giới literally (case insensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
u modifier: unicode. Pattern strings are treated as UTF-16. Also causes escape sequences to match unicode characters
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gmui

Test String