Use lineBoundary to match the start or end of a line of text.

Create a string with a newline character. Create a pattern that matches letters following the start of a new line.

txt = "This is line one." + newline + "Here is line two.";
pat = lineBoundary + lettersPattern;

Extract the pattern.

firstWord = extract(txt,pat)

firstWord = 2x1 string
    "This"
    "Here"

Use the "start" option for lineBoundary to match the specified endpoint of a line.

Create a string with newline characters. Create a pattern that matches any characters between two "start" boundaries of lines.

txt = "This is line one." + newline + "Here is line two." + newline + "Last but not least.";
pat = lineBoundary("start") + wildcardPattern(1,inf) + lineBoundary("start");

Extract the pattern.

extract(txt,pat)

ans = 2x1 string
    "This is line one...."
    "Here is line two...."

Use the ~ operator to negate lineBoundary. This matches boundaries between two characters when neither is a newline character.

Create a string with a newline character. Create a pattern that matches letters that are neither at the start nor end of a line of text.

txt = "This is line one" + newline + "Here is line two";
pat = ~lineBoundary + lettersPattern + ~lineBoundary;

Extract the pattern.

firstWord = extract(txt,pat)

firstWord = 8x1 string
    "his"
    "is"
    "line"
    "on"
    "ere"
    "is"
    "line"
    "tw"