regex101: create or change one optional part to another

Pro

Comment: skip match process when S exist, or remove optional left most target T, capture left and right part of T. Use case: if N is not exist, modify/change T to N, or create N. If any create or modify write operated, appending output N to right. skip exist:

^start of line

Negative Lookahead(?!.*S)

.*any character (except for line terminators), repeated any number of times

Sthe literal S (83₁₀ / 123₈ / 53₁₆) (case sensitive)

Comment: captrue prefix :

^start of line

Group 1(.*?)

.*?any character (except for line terminators), repeated any number of times

Comment: optional target to remove:

Non-Capturing Group(?:T|$)

1st AlternativeT

Tthe literal T (84₁₀ / 124₈ / 54₁₆) (case sensitive)

2nd Alternative$

$end of line

Comment: captrue postfix:

Group 2(.*?)

.*?any character (except for line terminators), repeated any number of times

$end of line

Comment: Test string: aa aaT Tcc aaTcc aaTbbTcc aaS aaTS TccS aaTccS Saa SaaT STcc SaaTcc Substitution: $1$2N Return: aaN aaN ccN aaccN aabbTccN aaS aaTS TccS aaTccS Saa SaaT STcc SaaTcc

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

25 tokens

Regular Expression

(?#skip match process when S exist, or remove optional left most target T, capture left and right part of T.
Use case: if N is not exist, modify/change T to N, or create N. If any create or modify write operated, appending output N to right.
skip exist:)^(?!.*S)(?#
captrue prefix :)^(.*?)(?#
optional target to remove:)(?:T|$)(?#
captrue postfix:)(.*?)$(?#
Test string:
aa
aaT
Tcc
aaTcc
aaTbbTcc
aaS
aaTS
TccS
aaTccS
Saa
SaaT
STcc
SaaTcc
Substitution: $1$2N
Return:
aaN
aaN
ccN
aaccN
aabbTccN
aaS
aaTS
TccS
aaTccS
Saa
SaaT
STcc
SaaTcc)

Regular Expressions 101

Regular Expression

Unit Tests