Regular Expressions 101

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/
^(?:([*_`]|\*\*|\[){0,1}[a-z]+)(.*)
/
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^ asserts position at start of a line
Non-capturing group
(?:([*_`]|\*\*|\[){0,1}[a-z]+)
1st Capturing Group
([*_`]|\*\*|\[){0,1}
{0,1} matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
1st Alternative
[*_`]
Match a single character present in the list below
[*_`]
*_`
matches a single character in the list *_` (case insensitive)
2nd Alternative
\*\*
\* matches the character * with index 4210 (2A16 or 528) literally (case insensitive)
\* matches the character * with index 4210 (2A16 or 528) literally (case insensitive)
3rd Alternative
\[
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case insensitive)
Match a single character present in the list below
[a-z]
+ 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 insensitive)
2nd Capturing Group
(.*)
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
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Test String