Regular Expressions 101

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/
^(.{14}).*\s^(?=.{0,14}^\1.*)
/
gms
^ asserts position at start of a line
1st Capturing Group
(.{14})
.
matches any character
{14} matches the previous token exactly 14 times
.
matches any character
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
^ asserts position at start of a line
Positive Lookahead
(?=.{0,14}^\1.*)
Assert that the Regex below matches
.
matches any character
{0,14} matches the previous token between 0 and 14 times, as many times as possible, giving back as needed (greedy)
^ asserts position at start of a line
\1 matches the same text as most recently matched by the 1st capturing group
.
matches any character
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
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)
s modifier: single line. Dot matches newline characters
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gms

Test String

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for Python, please visit: https://docs.python.org/3/library/re.html