fix: preserve unrecognized escape sequences in string literals

[chenjunwenhao]

Problem

QLStringUtils.parseStringEscapeStartEnd() silently drops both the backslash and the following character for unrecognized escape sequences like \d, \w, \s, etc.

For example, the string literal '(\d*)ch' produces (*)ch instead of (\d*)ch, which breaks regex patterns embedded in string literals (reported in #335).

Root Cause

In QLStringUtils.java, the escape state's switch statement only handles recognized escape characters (b, t, n, f, r, ", ', \, $). There is no default case, so when an unrecognized character follows a backslash, neither the backslash nor the character is appended to the result.

Fix

Added a default case to the escape state switch that preserves the backslash and the character as-is:

default:
    result.append('\\');
    result.append(cur);
    break;

This means:

• \d → \d (preserved for regex use)
• \w → \w (preserved for regex use)
• \n → newline (still works as before)
• \\ → \ (still works as before)

Changes

1. QLStringUtils.java — Added default case in escape state switch
2. QLStringUtilsTest.java — New unit test class with 20+ test cases covering recognized, unrecognized, and mixed escape sequences
3. string_escape.ql — Extended test suite with unrecognized escape and regex pattern tests
4. README-source.adoc / README-EN-source.adoc — Added "String Escape Sequences" documentation section listing all supported escapes

Testing

• 20+ new JUnit tests for QLStringUtils covering all recognized escapes, all common unrecognized escapes (\d, \w, \s, \D, \W, \S, \(, \., \*, \+, \1), complex regex patterns, and mixed scenarios
• Extended string_escape.ql test suite with regex pattern assertions