regex101: Python Match Workspace

Explanation

Pro

Positive Lookahead(?=(?:.*^From\s*:\s*)(.*?$)|)

1st Alternative(?:.*^From\s*:\s*)(.*?$)

Non-Capturing Group(?:.*^From\s*:\s*)

.*any Unicode character, repeated any number of times

^start of line

Fromthe literal text From (case sensitive)

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

:the literal : (58₁₀ / 72₈ / 3A₁₆)

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

Group 1(.*?$)

.*?any Unicode character, repeated any number of times

$end of line

2nd Alternative — matches an empty string

Positive Lookahead(?=(?:.*^Sent\s*:\s*)(.*?$)|)

1st Alternative(?:.*^Sent\s*:\s*)(.*?$)

Non-Capturing Group(?:.*^Sent\s*:\s*)

.*any Unicode character, repeated any number of times

^start of line

Sentthe literal text Sent (case sensitive)

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

:the literal : (58₁₀ / 72₈ / 3A₁₆)

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

Group 2(.*?$)

.*?any Unicode character, repeated any number of times

$end of line

2nd Alternative — matches an empty string

Positive Lookahead(?=(?:.*^To\s*:\s*)(.*?$)|)

1st Alternative(?:.*^To\s*:\s*)(.*?$)

Non-Capturing Group(?:.*^To\s*:\s*)

.*any Unicode character, repeated any number of times

^start of line

Tothe literal text To (case sensitive)

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

:the literal : (58₁₀ / 72₈ / 3A₁₆)

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

Group 3(.*?$)

.*?any Unicode character, repeated any number of times

$end of line

2nd Alternative — matches an empty string

Positive Lookahead(?=(?:.*^Subject\s*:\s*)(.*?$)|)

1st Alternative(?:.*^Subject\s*:\s*)(.*?$)

Non-Capturing Group(?:.*^Subject\s*:\s*)

.*any Unicode character, repeated any number of times

^start of line

Subjectthe literal text Subject (case sensitive)

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

:the literal : (58₁₀ / 72₈ / 3A₁₆)

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

Group 4(.*?$)

.*?any Unicode character, repeated any number of times

$end of line

2nd Alternative — matches an empty string

Global pattern flags

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

s modifier: single line. Allows a dot to match line terminators

Quick Reference

25 tokens

Regular Expression

(?=(?:.*^From\s*:\s*)(.*?$)|)(?=(?:.*^Sent\s*:\s*)(.*?$)|)(?=(?:.*^To\s*:\s*)(.*?$)|)(?=(?:.*^Subject\s*:\s*)(.*?$)|)

New

To: ...   
Sent: ...   
this text has no no "from" label
Subject: ...

Substitution

To: ...   
Sent: ...   
this text has no no "from" label
Subject: ...