Source

File: wp-includes/class-wp-query.php

public function parse_tax_query( &$q ) {
		if ( ! empty( $q['tax_query'] ) && is_array( $q['tax_query'] ) ) {
			$tax_query = $q['tax_query'];
		} else {
			$tax_query = array();
		}

		if ( ! empty( $q['taxonomy'] ) && ! empty( $q['term'] ) ) {
			$tax_query[] = array(
				'taxonomy' => $q['taxonomy'],
				'terms'    => array( $q['term'] ),
				'field'    => 'slug',
			);
		}

		foreach ( get_taxonomies( array(), 'objects' ) as $taxonomy => $t ) {
			if ( 'post_tag' === $taxonomy ) {
				continue; // Handled further down in the $q['tag'] block.
			}

			if ( $t->query_var && ! empty( $q[ $t->query_var ] ) ) {
				$tax_query_defaults = array(
					'taxonomy' => $taxonomy,
					'field'    => 'slug',
				);

				if ( isset( $t->rewrite['hierarchical'] ) && $t->rewrite['hierarchical'] ) {
					$q[ $t->query_var ] = wp_basename( $q[ $t->query_var ] );
				}

				$term = $q[ $t->query_var ];

				if ( is_array( $term ) ) {
					$term = implode( ',', $term );
				}

				if ( strpos( $term, '+' ) !== false ) {
					$terms = preg_split( '/[+]+/', $term );
					foreach ( $terms as $term ) {
						$tax_query[] = array_merge(
							$tax_query_defaults,
							array(
								'terms' => array( $term ),
							)
						);
					}
				} else {
					$tax_query[] = array_merge(
						$tax_query_defaults,
						array(
							'terms' => preg_split( '/[,]+/', $term ),
						)
					);
				}
			}
		}

		// If query string 'cat' is an array, implode it.
		if ( is_array( $q['cat'] ) ) {
			$q['cat'] = implode( ',', $q['cat'] );
		}

		// Category stuff.

		if ( ! empty( $q['cat'] ) && ! $this->is_singular ) {
			$cat_in     = array();
			$cat_not_in = array();

			$cat_array = preg_split( '/[,\s]+/', urldecode( $q['cat'] ) );
			$cat_array = array_map( 'intval', $cat_array );
			$q['cat']  = implode( ',', $cat_array );

			foreach ( $cat_array as $cat ) {
				if ( $cat > 0 ) {
					$cat_in[] = $cat;
				} elseif ( $cat < 0 ) {
					$cat_not_in[] = abs( $cat );
				}
			}

			if ( ! empty( $cat_in ) ) {
				$tax_query[] = array(
					'taxonomy'         => 'category',
					'terms'            => $cat_in,
					'field'            => 'term_id',
					'include_children' => true,
				);
			}

			if ( ! empty( $cat_not_in ) ) {
				$tax_query[] = array(
					'taxonomy'         => 'category',
					'terms'            => $cat_not_in,
					'field'            => 'term_id',
					'operator'         => 'NOT IN',
					'include_children' => true,
				);
			}
			unset( $cat_array, $cat_in, $cat_not_in );
		}

		if ( ! empty( $q['category__and'] ) && 1 === count( (array) $q['category__and'] ) ) {
			$q['category__and'] = (array) $q['category__and'];
			if ( ! isset( $q['category__in'] ) ) {
				$q['category__in'] = array();
			}
			$q['category__in'][] = absint( reset( $q['category__and'] ) );
			unset( $q['category__and'] );
		}

		if ( ! empty( $q['category__in'] ) ) {
			$q['category__in'] = array_map( 'absint', array_unique( (array) $q['category__in'] ) );
			$tax_query[]       = array(
				'taxonomy'         => 'category',
				'terms'            => $q['category__in'],
				'field'            => 'term_id',
				'include_children' => false,
			);
		}

		if ( ! empty( $q['category__not_in'] ) ) {
			$q['category__not_in'] = array_map( 'absint', array_unique( (array) $q['category__not_in'] ) );
			$tax_query[]           = array(
				'taxonomy'         => 'category',
				'terms'            => $q['category__not_in'],
				'operator'         => 'NOT IN',
				'include_children' => false,
			);
		}

		if ( ! empty( $q['category__and'] ) ) {
			$q['category__and'] = array_map( 'absint', array_unique( (array) $q['category__and'] ) );
			$tax_query[]        = array(
				'taxonomy'         => 'category',
				'terms'            => $q['category__and'],
				'field'            => 'term_id',
				'operator'         => 'AND',
				'include_children' => false,
			);
		}

		// If query string 'tag' is array, implode it.
		if ( is_array( $q['tag'] ) ) {
			$q['tag'] = implode( ',', $q['tag'] );
		}

		// Tag stuff.

		if ( '' !== $q['tag'] && ! $this->is_singular && $this->query_vars_changed ) {
			if ( strpos( $q['tag'], ',' ) !== false ) {
				$tags = preg_split( '/[,\r\n\t ]+/', $q['tag'] );
				foreach ( (array) $tags as $tag ) {
					$tag                 = sanitize_term_field( 'slug', $tag, 0, 'post_tag', 'db' );
					$q['tag_slug__in'][] = $tag;
				}
			} elseif ( preg_match( '/[+\r\n\t ]+/', $q['tag'] ) || ! empty( $q['cat'] ) ) {
				$tags = preg_split( '/[+\r\n\t ]+/', $q['tag'] );
				foreach ( (array) $tags as $tag ) {
					$tag                  = sanitize_term_field( 'slug', $tag, 0, 'post_tag', 'db' );
					$q['tag_slug__and'][] = $tag;
				}
			} else {
				$q['tag']            = sanitize_term_field( 'slug', $q['tag'], 0, 'post_tag', 'db' );
				$q['tag_slug__in'][] = $q['tag'];
			}
		}

		if ( ! empty( $q['tag_id'] ) ) {
			$q['tag_id'] = absint( $q['tag_id'] );
			$tax_query[] = array(
				'taxonomy' => 'post_tag',
				'terms'    => $q['tag_id'],
			);
		}

		if ( ! empty( $q['tag__in'] ) ) {
			$q['tag__in'] = array_map( 'absint', array_unique( (array) $q['tag__in'] ) );
			$tax_query[]  = array(
				'taxonomy' => 'post_tag',
				'terms'    => $q['tag__in'],
			);
		}

		if ( ! empty( $q['tag__not_in'] ) ) {
			$q['tag__not_in'] = array_map( 'absint', array_unique( (array) $q['tag__not_in'] ) );
			$tax_query[]      = array(
				'taxonomy' => 'post_tag',
				'terms'    => $q['tag__not_in'],
				'operator' => 'NOT IN',
			);
		}

		if ( ! empty( $q['tag__and'] ) ) {
			$q['tag__and'] = array_map( 'absint', array_unique( (array) $q['tag__and'] ) );
			$tax_query[]   = array(
				'taxonomy' => 'post_tag',
				'terms'    => $q['tag__and'],
				'operator' => 'AND',
			);
		}

		if ( ! empty( $q['tag_slug__in'] ) ) {
			$q['tag_slug__in'] = array_map( 'sanitize_title_for_query', array_unique( (array) $q['tag_slug__in'] ) );
			$tax_query[]       = array(
				'taxonomy' => 'post_tag',
				'terms'    => $q['tag_slug__in'],
				'field'    => 'slug',
			);
		}

		if ( ! empty( $q['tag_slug__and'] ) ) {
			$q['tag_slug__and'] = array_map( 'sanitize_title_for_query', array_unique( (array) $q['tag_slug__and'] ) );
			$tax_query[]        = array(
				'taxonomy' => 'post_tag',
				'terms'    => $q['tag_slug__and'],
				'field'    => 'slug',
				'operator' => 'AND',
			);
		}

		$this->tax_query = new WP_Tax_Query( $tax_query );

		/**
		 * Fires after taxonomy-related query vars have been parsed.
		 *
		 * @since 3.7.0
		 *
		 * @param WP_Query $this The WP_Query instance.
		 */
		do_action( 'parse_tax_query', $this );
	}