slake-school-service/app/Services/Crawl/ArxivTextNormalizer.php

<?php

namespace App\Services\Crawl;

/**
 * 将 arXiv Atom/API 摘要中的 LaTeX 行内公式转为接近官网展示的纯文本。
 */
class ArxivTextNormalizer
{
    public static function normalize(?string $text): ?string
    {
        if ($text === null || $text === '') {
            return $text;
        }

        $text = html_entity_decode($text, ENT_QUOTES | ENT_HTML5, 'UTF-8');
        $text = preg_replace('/\s+/u', ' ', $text) ?? $text;

        // 去掉 HTML 降级时可能残留的检索高亮标记
        $text = preg_replace('/\s*…\s*/u', ' ', $text) ?? $text;

        $text = preg_replace_callback(
            '/\$\$([^$]+)\$\$/s',
            static fn (array $m): string => self::latexFragmentToPlain($m[1]),
            $text
        ) ?? $text;

        $text = preg_replace_callback(
            '/\$([^$]+)\$/',
            static fn (array $m): string => self::latexFragmentToPlain($m[1]),
            $text
        ) ?? $text;

        $text = preg_replace('/\\\\(mathrm|mathbf|mathit|text|textbf|textit)\{([^}]*)\}/', '$2', $text) ?? $text;
        $text = str_replace(['\\', '{', '}'], '', $text);
        $text = preg_replace('/\s+/u', ' ', $text) ?? $text;

        return trim($text);
    }

    protected static function latexFragmentToPlain(string $latex): string
    {
        $s = trim($latex);

        $s = preg_replace_callback(
            '/\^\{([^}]*)\}/',
            static fn (array $m): string => self::formatExponent($m[1]),
            $s
        ) ?? $s;

        $s = preg_replace_callback(
            '/\^(-?\d+)/',
            static fn (array $m): string => self::formatExponent($m[1]),
            $s
        ) ?? $s;

        $s = preg_replace('/_\{([^}]+)\}/', '$1', $s) ?? $s;
        $s = preg_replace('/_([a-zA-Z0-9]+)/', '$1', $s) ?? $s;

        $s = preg_replace('/\\\\(mathrm|mathbf|mathit|text)\{([^}]*)\}/', '$2', $s) ?? $s;
        $s = str_replace(['\\', '{', '}'], '', $s);

        return $s;
    }

    protected static function formatExponent(string $exp): string
    {
        if (preg_match('/^-\d+$/', $exp)) {
            return '−'.substr($exp, 1);
        }

        return $exp;
    }
}