regex101: PCRE (Legacy) Substitution Workspace

Group 1(android:layout_marginStart="(.*?)")

android:layout_marginStart="the literal text android:layout_marginStart=" (case sensitive)

Group 2(.*?)

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

"the literal " (34₁₀ / 42₈ / 22₁₆)

Non-Capturing Group(?:(?!layout_marginLeft).)*

*repeats the group contents below any number of times:

Negative Lookahead(?!layout_marginLeft)

layout_marginLeftthe literal text layout_marginLeft (case sensitive)

.any character (except for line terminators)

Positive Lookahead(?=/>)

/>the literal text />

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

(android:layout_marginStart="(.*?)")(?:(?!layout_marginLeft).)*(?=/>)

New

<androidx.constraintlayout.widget.Barrier
        android:id="@+id/barrier6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="12dp"/>

<androidx.constraintlayout.widget.Barrier
        android:id="@+id/barrier6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="15px"/>

<androidx.constraintlayout.widget.Barrier
        android:id="@+id/barrier6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="28em"
		android:layout_marginLeft="28em"/>

Substitution

\1\n\t\tandroid:layout_marginLeft="\2"