"negative lookahead" meaning in English

{
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "negative",
        "3": "lookahead"
      },
      "expansion": "negative + lookahead",
      "name": "compound"
    }
  ],
  "etymology_text": "From negative + lookahead.",
  "forms": [
    {
      "form": "negative lookaheads",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "negative lookahead (countable and uncountable, plural negative lookaheads)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Regular expressions",
          "orig": "en:Regular expressions",
          "parents": [
            "Computing",
            "Programming",
            "Technology",
            "Software engineering",
            "All topics",
            "Computer science",
            "Engineering",
            "Software",
            "Fundamental",
            "Sciences",
            "Applied sciences",
            "Media",
            "Communication"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "2012 August, Jan Goyvaerts, Steven Levithan, Regular Expressions Cookbook:",
          "text": "Negative lookahead works just like positive lookahead, except that whereas positive lookahead matches when the regex inside the lookahead matches, negative lookahead matches when the regex inside the lookahead failed to match.",
          "type": "quote"
        }
      ],
      "glosses": [
        "The analysis of the subsequent characters absence before the regular expression is matched (symbol: ?!)."
      ],
      "hypernyms": [
        {
          "sense": "lookaround",
          "word": "lookahead"
        }
      ],
      "id": "en-negative_lookahead-en-noun-Ejm9jnPD",
      "links": [
        [
          "regular expression",
          "regular expression"
        ],
        [
          "analysis",
          "analysis"
        ],
        [
          "character",
          "character"
        ]
      ],
      "qualifier": "regular expressions",
      "raw_glosses": [
        "(regular expressions) The analysis of the subsequent characters absence before the regular expression is matched (symbol: ?!)."
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "word": "negative lookahead"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "negative",
        "3": "lookahead"
      },
      "expansion": "negative + lookahead",
      "name": "compound"
    }
  ],
  "etymology_text": "From negative + lookahead.",
  "forms": [
    {
      "form": "negative lookaheads",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "negative lookahead (countable and uncountable, plural negative lookaheads)",
      "name": "en-noun"
    }
  ],
  "hypernyms": [
    {
      "sense": "lookaround",
      "word": "lookahead"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English compound terms",
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Regular expressions"
      ],
      "examples": [
        {
          "ref": "2012 August, Jan Goyvaerts, Steven Levithan, Regular Expressions Cookbook:",
          "text": "Negative lookahead works just like positive lookahead, except that whereas positive lookahead matches when the regex inside the lookahead matches, negative lookahead matches when the regex inside the lookahead failed to match.",
          "type": "quote"
        }
      ],
      "glosses": [
        "The analysis of the subsequent characters absence before the regular expression is matched (symbol: ?!)."
      ],
      "links": [
        [
          "regular expression",
          "regular expression"
        ],
        [
          "analysis",
          "analysis"
        ],
        [
          "character",
          "character"
        ]
      ],
      "qualifier": "regular expressions",
      "raw_glosses": [
        "(regular expressions) The analysis of the subsequent characters absence before the regular expression is matched (symbol: ?!)."
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "word": "negative lookahead"
}