Regular Expressions 101

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/
a(?:\[\d])?\.b(?:\[\d])?\.c(?:\[\d])?\.d(?:\[\d])?\.e(?:\[\d])?
/
gm
a matches the character a literally (case sensitive)
Non-capturing group
(?:\[\d])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\[ matches the character [ literally (case sensitive)
\d matches a digit (equivalent to [0-9])
] matches the character ] literally (case sensitive)
\. matches the character . literally (case sensitive)
b matches the character b literally (case sensitive)
Non-capturing group
(?:\[\d])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\[ matches the character [ literally (case sensitive)
\d matches a digit (equivalent to [0-9])
] matches the character ] literally (case sensitive)
\. matches the character . literally (case sensitive)
c matches the character c literally (case sensitive)
Non-capturing group
(?:\[\d])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\[ matches the character [ literally (case sensitive)
\d matches a digit (equivalent to [0-9])
] matches the character ] literally (case sensitive)
\. matches the character . literally (case sensitive)
d matches the character d literally (case sensitive)
Non-capturing group
(?:\[\d])?
\. matches the character . literally (case sensitive)
e matches the character e literally (case sensitive)
Non-capturing group
(?:\[\d])?
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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String