Regular Expressions 101

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/
(?<=Client Name: )(.+)(?=\\r\\nClient Country)
/
gm
Positive Lookbehind
(?<=Client Name: )
Assert that the Regex below matches
Client Name:
matches the characters Client Name: literally (case sensitive)
1st Capturing Group
(.+)
.
matches any character (except for line terminators)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Positive Lookahead
(?=\\r\\nClient Country)
Assert that the Regex below matches
\\ matches the character \ with index 9210 (5C16 or 1348) literally (case sensitive)
r matches the character r with index 11410 (7216 or 1628) literally (case sensitive)
\\ matches the character \ with index 9210 (5C16 or 1348) literally (case sensitive)
nClient Country
matches the characters nClient Country literally (case sensitive)
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