Regular Expressions 101

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"
^[a-z]*rust[a-z0-9 ]+$
"
gm
^ asserts position at start of a line
Match a single character present in the list below
[a-z]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
rust
matches the characters rust literally (case sensitive)
Match a single character present in the list below
[a-z0-9 ]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
$ asserts position at the end of a line
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)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
gm

Test String