How custom the text in flutter:

stylizableText.dart

import 'package:flutter/widgets.dart';

final regExpPattern = r'\[([^\]]+)\]\(([^\s\)]+)\)';
final regExp = RegExp(regExpPattern);

const String BOLD = 'B';
const String ITALIC = 'I';
const String COLORED = 'C';
const String BOLD_AND_COLORED = 'BC';
const String ITALIC_AND_COLORED = 'IC';
const String ITALIC_AND_BOLD = 'IB';

class StylizedText extends StatelessWidget {
  final String text;
  final Color color;
  final Color stylizedTextColor;
  final double fontSize;
  final FontWeight fontWeight;
  final TextAlign textAlign;

  const StylizedText({
    @required this.text,
    @required this.color,
    this.stylizedTextColor,
    @required this.fontSize,
    @required this.fontWeight,
    this.textAlign = TextAlign.start,
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return RichText(
      text: TextSpan(
        children: _createTextSpans(text),
      ),
      textAlign: textAlign,
    );
  }

  List<TextSpan> _createTextSpans(String content) {
    List<TextSpan> result = [];
    List<String> splitContentList = _generateElements(content);
    for (String text in splitContentList) {
      result.add(getStylizedText(text));
    }
    return result;
  }

  TextSpan getStylizedText(String text) {
    final match = regExp.firstMatch(text);
    TextStyle textStyle = TextStyle(
      color: color,
      fontSize: fontSize,
      fontWeight: fontWeight,
    );
    if (match?.groupCount == 2) {
      text = match.group(1);
      String style = _getStyleName(match.group(2));
      switch (style) {
        case BOLD:
          textStyle = TextStyle(
            color: color,
            fontSize: fontSize,
            fontWeight: FontWeight.bold,
          );
          break;
        case ITALIC:
          textStyle = TextStyle(
            color: color,
            fontSize: fontSize,
            fontWeight: fontWeight,
            fontStyle: FontStyle.italic,
          );
          break;
        case COLORED:
          textStyle = TextStyle(
            color: _stylizedTextColor(),
            fontSize: fontSize,
            fontWeight: fontWeight,
          );
          break;
        case BOLD_AND_COLORED:
          textStyle = TextStyle(
            color: _stylizedTextColor(),
            fontSize: fontSize,
            fontWeight: FontWeight.bold,
          );
          break;
        case ITALIC_AND_COLORED:
          textStyle = TextStyle(
            color: _stylizedTextColor(),
            fontSize: fontSize,
            fontWeight: fontWeight,
            fontStyle: FontStyle.italic,
          );
          break;
        case ITALIC_AND_BOLD:
          textStyle = TextStyle(
            color: color,
            fontSize: fontSize,
            fontWeight: FontWeight.bold,
            fontStyle: FontStyle.italic,
          );
          break;
      }
    }
    return TextSpan(text: text, style: textStyle);
  }

  Color _stylizedTextColor() => stylizedTextColor != null ? stylizedTextColor : color;
}

_getStyleName(String styleName) {
  if (styleName == 'CB') {
    return styleName = BOLD_AND_COLORED;
  } else if (styleName == 'CI') {
    return styleName = ITALIC_AND_COLORED;
  } else if (styleName == 'BI') {
    return styleName = ITALIC_AND_BOLD;
  } else {
    return styleName;
  }
}

List<String> _generateElements(String text) {
  if (text.isEmpty) return [];

  List<String> elements = List();

  final matches = regExp.allMatches(text);

  if (matches.isEmpty) {
    elements.add(
      text,
    );
  } else {
    var index = 0;
    matches.forEach((match) {
      if (match.start != 0) {
        elements.add(
          text.substring(index, match.start),
        );
      }
      elements.add(
        match.group(0),
      );
      index = match.end;
    });

    if (index < text.length) {
      elements.add(
        text.substring(index),
      );
    }
  }
  return elements;
}