Regular Expressions 101

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/
^field\s*b\s*\=\s*(\'|\s)(?<fielda>\d+)
/
gm
^ asserts position at start of a line
field
matches the characters field literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
b matches the character b with index 9810 (6216 or 1428) literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\= matches the character = with index 6110 (3D16 or 758) literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
1st Capturing Group
(\'|\s)
1st Alternative
\'
\' matches the character ' with index 3910 (2716 or 478) literally (case sensitive)
2nd Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
Named Capture Group fielda
(?<fielda>\d+)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one 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)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String