How to make specific text of a string bold if i marked in some way in react native?

You could do something like the below code.

Step-wise details:

1. Split the text string around ** to create an array arr of string.
2. The string inside the **...** will be at odd indices in arr array.
3. So, wrap strings at odd indices with bold-styled Text component and other strings at even indices(i.e. which are outside the **...** block) with simple Text component.
4. Append these Text components to an array.
5. Render this array of Text components in another Text component to display all of them as a single component on the same line.

import React from "react";
import { StyleSheet, Text, View } from "react-native";

const styles = StyleSheet.create({
  app: {
    width: "100%"
  },
  bold: {
    fontWeight: "bold"
  }
});

function App() {
  const text =
    "**Lorem** ipsum dolor sit, *amet consectetur **adipisicing** elit.";

  function getSmartBold() {
    // Split the text around **
    const arr = text.split("**");

    // Here we will store an array of Text components
    const newTextArr = [];

    // Loop over split text
    arr.forEach((element, index) => {
      // If its an odd element then it is inside **...** block
      if (index % 2 !== 0) {
        // Wrap with bold text style
        const newElement = <Text style={styles.bold}>{element}</Text>;
        newTextArr.push(newElement);
      } else {
        // Simple Text
        const newElement = <Text>{element}</Text>;
        newTextArr.push(newElement);
      }
    });

    return newTextArr;
  }

  return (
    <View style={styles.app}>
      <Text>{getSmartBold()}</Text>
    </View>
  );
}

export default App;

CodeSandbox link: https://codesandbox.io/s/happy-cori-oex9kg?file=/src/App.js

Edit: If you want the bold-text functionality to be smarter, you can check for stray texts which have dangling **. Code for the same:

import React from "react";
import { StyleSheet, Text, View } from "react-native";

const styles = StyleSheet.create({
  app: {
    width: "100%"
  },
  bold: {
    fontWeight: "bold"
  }
});

function getSmartBoldText(text, boldWrapperText = "**") {
  // Split the text around **
  const splitStringArr = text.split(boldWrapperText);

  console.debug(`splitStringArr = `, splitStringArr);

  // Here we will store an array of Text components
  const textComponentsArr = [];

  // Loop over split text
  splitStringArr.forEach((string, index) => {
    // If its an odd element then it is inside **...** block
    // And is not a word surrounded by stray ** (without any closing)
    if (index % 2 !== 0 && index !== splitStringArr.length - 1) {
      // Wrap with bold text style
      const boldText = <Text style={styles.bold}>{string}</Text>;
      textComponentsArr.push(boldText);
    } else {
      // Simple Text

      // If it's stray element, append ** to it
      if (index === splitStringArr.length - 2) {
        string = string + boldWrapperText;
      }

      const normalText = <Text>{string}</Text>;
      textComponentsArr.push(normalText);
    }
  });

  return textComponentsArr;
}

function App() {
  const text =
    "**Lorem ipsum dolor sit, *amet consectetur **adipisicing **elit.";

  return (
    <View style={styles.app}>
      <Text>{getSmartBoldText(text, "**")}</Text>
    </View>
  );
}

export default App;

CodeSandbox link for smarter bold-text functionality: https://codesandbox.io/s/boring-haslett-238nt3?file=/src/App.js