regex101: Python Substitution Workspace

(?mx)applies the following effective flags to the remainder of the pattern: gmx

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

Comment: MULTILINE mode on

^start of line

Comment: start of string

Non-Capturing Group(?: # non-capturing group start EXT\s*\|\|\s*(?:RE:\s*EXTERNAL:\s*RE:|FW:)? # EXT || or EXT || RE: EXTERNAL: RE: or EXT || FW: | # or SV:\s*\[EXTERNAL]# SV: [EXTERNAL] )

1st Alternative # non-capturing group start EXT\s*\|\|\s*(?:RE:\s*EXTERNAL:\s*RE:|FW:)? # EXT || or EXT || RE: EXTERNAL: RE: or EXT || FW:

Comment: non-capturing group start

EXTthe literal text EXT (case sensitive)

\s*a Unicode whitespace character, repeated any number of times

\|the literal | (124₁₀ / 174₈ / 7C₁₆)

\s*a Unicode whitespace character, repeated any number of times

Non-Capturing Group(?:RE:\s*EXTERNAL:\s*RE:|FW:)?

?repeats the group contents below at most once:

1st AlternativeRE:\s*EXTERNAL:\s*RE:

RE:the literal text RE: (case sensitive)

\s*a Unicode whitespace character, repeated any number of times

EXTERNAL:the literal text EXTERNAL: (case sensitive)

\s*a Unicode whitespace character, repeated any number of times

RE:the literal text RE: (case sensitive)

2nd AlternativeFW:

FW:the literal text FW: (case sensitive)

Comment: EXT || or EXT || RE: EXTERNAL: RE: or EXT || FW:

2nd Alternative # or SV:\s*\[EXTERNAL]# SV: [EXTERNAL]

Comment: or

SV:the literal text SV: (case sensitive)

\s*a Unicode whitespace character, repeated any number of times

\[the literal [ (91₁₀ / 133₈ / 5B₁₆)

EXTERNAL]the literal text EXTERNAL] (case sensitive)

Comment: SV: [EXTERNAL]

Comment: non-capturing group end

\s*a Unicode whitespace character, repeated any number of times

Comment: zero or more whitespaces

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Search reference

Regular Expression

(?mx)  # MULTILINE mode on
^                   # start of string
(?:                 # non-capturing group start
   EXT\s*\|\|\s*(?:RE:\s*EXTERNAL:\s*RE:|FW:)? # EXT || or EXT || RE: EXTERNAL: RE: or EXT || FW:
 |                  # or
   SV:\s*\[EXTERNAL]# SV: [EXTERNAL]
)                   # non-capturing group end
\s*                 # zero or more whitespaces

Unit Tests

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